kye

I wouldn't buy a Mac right now. I'd wait until after June if possible. I think there will be new M4 macs, most likely a new Mac Studio, Mini, MacBook Pro, and maybe a new iMac. Or, if you do get the "older" model, get it at a reduced price.

The only way to upgrade the GPU on a MBP is to buy a new MBP for another $4k.  Not the best plan unless you're independently wealthy - and if so, then you might as well just spend the extra $300 for the better chip now.  😃

BM has sample files for download.  They're on these pages under the "Generation 5 Color Science" heading (different files are available for download on each page):
https://www.blackmagicdesign.com/products/blackmagicursacine
https://www.blackmagicdesign.com/products/blackmagicursaminipro

there is a cost effect ratio here too. other than magic lantern cameras, uncompressed raw cameras are extremely expensive, like the Alexa family. on the other hand, a 60p camera with a low end codec can be very cheap. doing action packed shots, the latter one will be more suitable. try a 24p thin codec camera, you can hardly see clearly anything once the camera is panning, tilting, or doing other complexing movements. 

The above is exactly why I say there are asterisks and "it depends" in the answer.
Will either computer be able to play back 12K braw in Resolve without dropping frames?  As long as the storage is fast enough, absolutely.  Will it be able to play back in Premiere without dropping frames?  🤷‍♂️ FCP? 🤷‍♂️
As I said before, my M2 Max (the weaker version) w/ 64GB is just able to play back 8K raw from my R5 in Resolve.  If I put on a grade where I tweak a few raw options, add FilmConvert Nitrate, and tweak some curves/color warper stuff afterward, I still get 23.98 on it as long as I'm not doing anything else in the background.  Canon raw is a nightmare codec in terms of performance.  Braw is easier.
One other thing to consider is that I'm sure that the local Apple store (if there is one) has some M3 Max models on the showroom floor.  If you're nice about it, they might be willing to at least put FCP on one (or let you download Resolve) to load a sample project file and some footage.  Since the UM12K has been out for a long time, I'm sure some nice reviewer/youtuber has put some raw files online for you to download.  You could just give it a try with one of your sample grades to understand if it performs as needed.

It would be great to be able to give a straight answer, but there are too many variables.
Even something like the SSD option of the Mac model chosen might mean that it can or cannot do something.
Going to Resolve, there are literally dozens of variables involved.  Even if it's just 12K footage on a 4K timeline with a LUT - will Resolve be using Linear or Tetrahedral interpolation for the LUT?  This matters as it's extra processing power required, etc.

Further to what @eatstoomuchjam said, I think there are only three types of performance:
it can't play ungraded footage realtime (ie, you can't edit with it) it can play footage realtime but can't do it with a heavy grade (ie, you can do some colour grading realtime) it can play footage realtime with a heavy grade (ie, it's reliable and you can colour grade live in front of a client) The spec required for level 2 should be researchable and BM would have made this pretty efficient I would imagine.  The spec for level 3 is a "how long is a piece of string" question, depending on a ton of variables including timeline resolution and only you can answer it, but if you're going to be doing lots of heavy things like NR and spatial treatments on a 4K timeline then you're probably well into Ultra / Studio territory.

Here is an answer from a pro colorist / editor. It will give you a perspective and a cool tip how to edit any footage on a not very powerful computer.
https://www.youtube.com/watch?v=olvTKnLJCJc
https://www.youtube.com/watch?v=lTes9KkwmME
According to another of his videos, 12K BRAW is so well optimized that it can play without a problem on a Macbook Pro 2017
https://www.youtube.com/watch?v=0ShXVTiGD80

There are a lot of asterisks and "it depends" in any answer to that, but overall, I'd say that if you're spending $4k on a MBP (which updating from the base RAM and SSD would do), if you plan to edit 12k, it might be worth the extra $300 to jump to the bigger chip.  It partly depends on how many effects/how much noise reduction, etc.  I'd also say that if you're like most people, if you're gonna spend $4k on a laptop, you're going to want it to last for a while.  You might factor that in too.
Speaking for myself, I bought the lower of the M2 Max chips in my MBP and it's just barely enough to handle 8k Canon raw in Resolve and once I add noise reduction to the clips that need it, export times get pretty slow (like 40 minutes for a 15 minute short).  Would having the system be 30% faster help a lot?  Not really.  30 minutes to export the clips wouldn't be life changing.
You might also consider whether M4 is coming soon - M2 was released in June 2022 and M3 in October 2023.  If the M4 is coming, it'll probably have about a 10-15% speed boost over M3 - that or you might be able to get a nice deal on an M3 at that time.  Depends on when you're planning to start shooting 12k, I suppose.  😁

I love the way Blaine constructs his videos. Zero fluff. Just tips and tricks the whole time.

Further to this, I watched a few past videos from Blaine yesterday and this video (while quite chaotic) gives a bunch of pretty interesting examples of how using a Film Emulation of some kind combined with some of the basic Resolve tools can give quick but very effective results:
Perhaps the most interesting thing about it is the way he treats Resolve..  he shows that if you know a few techniques then you can get in, make some good adjustments, get a great result, and get out, and move on with your day.
It's sort of a rare counter-example of the impression that Resolve is finicky and takes hours and hours, which almost all other colour grading videos give, but isn't true.
At it's most basic, you can just apply a LUT to every clip and then make basic adjustments in a node prior to the LUT on each clip and can get great results in a really quick way.  This is the way that film-makers grade when they want a result, rather than the way that colour grading YouTubers grade when they're making a YT video about some nuance or other.
The section from 5:00 on with the test shots shows that even if the shots weren't filmed well at all, you can get great results quickly with just a few quick techniques.

Cullen Kelly says that the grain and the halation are nicer than the previous separate OFX plugins, so that's promising if you're a connoisseur / picky 🙂 
 
 

Further to this, I watched a few past videos from Blaine yesterday and this video (while quite chaotic) gives a bunch of pretty interesting examples of how using a Film Emulation of some kind combined with some of the basic Resolve tools can give quick but very effective results:
Perhaps the most interesting thing about it is the way he treats Resolve..  he shows that if you know a few techniques then you can get in, make some good adjustments, get a great result, and get out, and move on with your day.
It's sort of a rare counter-example of the impression that Resolve is finicky and takes hours and hours, which almost all other colour grading videos give, but isn't true.
At it's most basic, you can just apply a LUT to every clip and then make basic adjustments in a node prior to the LUT on each clip and can get great results in a really quick way.  This is the way that film-makers grade when they want a result, rather than the way that colour grading YouTubers grade when they're making a YT video about some nuance or other.
The section from 5:00 on with the test shots shows that even if the shots weren't filmed well at all, you can get great results quickly with just a few quick techniques.

It's worth pointing out that the thermals might be the dominant factor here, considering that laptops will throttle down on their performance in order to manage overheating, so a few extra fans in the laptop can make more difference than which model of CPU / GPU you buy!

I've heard that the 12K files are very usable in terms of performance, but it will likely depend on what mode you're shooting in.  Most people aren't using the 12K at 12K - they're using it at 4K or 8K.
Regardless, Resolve has an incredible array of functionality to improve performance and enable real-time editing and even colour correction on lesser hardware.  This is a good overview:
 

I recommend FilmConvert or Dehancer if people like the overall film look but don't want to learn to colour grade - it's a quick and effective solution.
Resolve 19 has a new plugin called Film Look Creator included in the paid version that is like FilmConvert / Dehancer except it doesn't model specific film stocks but is designed to give a flexible overall film look.  For most people it would be better than FilmConvert or Dehancer just for creating a nice look and not having to learn colour management etc.

From the videos I've seen on Film Look Creator, I think subtractive saturation and split toning are the things that stuck at to me as most interesting - that and I'm going to need to spend a bit of time with their grain generator to see if I like the output.

Uhhh I know! Maybe this summer when I have more time! The advent of internal LUTs I think will help me on that journey. Seeing what folks have done with Dehancer has been motivating as well. 

Yep!
Hey - you should look into colour grading! 🙂 

Blaine Westropp is also killing it recently.

Well, I dislike HFR as much as the next guy, but the reason he's abandoning it is because of the ecosystemic/economic context within the movie industry which doesn't want to take visual risks with their production investment, not because Ang personally doesn't like HFR.
Aside from that, never been sure why he's enamored with the look.  I've read his rationale, but it still doesn't jibe.  HFR pulls movies way too close to visual "reality" and narrative film are stories.  Make 'em ups.  Pretend.  The nature of 24p's look is a huge asset. 

Slight digression from the topic, but unless you really need the flippy screen, you might consider the RX100 V over the ZV-1.  The V is even smaller, but weighs just a hair more (probably due to having more metal and less plastic in the body) (and I think the V is cheaper on the used market).  The flippy screen was just about the only difference between the two cameras.

Spenser Sakurai
Tenfold Productions
Robert Machado
- also deserve a mention.

My rule of thumb is that if I use an item on 5 or more shoots, buying (and reselling at the end) is cheaper than renting. That's probably fairly conservative too: I usually buy used, so when I sell I get 60-80% of the cost back. I've even sold cameras at a profit after 1-2 years.

If you're fighting the sun then yeah, serious horsepower definitely comes into the equation!