Seenematic

If you can provide me with a direct link to 32 GB RAM that is 100% compatible with a MacPro for $150, including shipping and taxes, I'd love to hear that. Please share all the Mac Pro 4.1's that you know for sale for £350. Because I haven't seen any.

That's a bargain. The RAM alone is almost worth that much. I still have the very same MacPro (bought it new back in 2008) and considered upgrading to a new iMac, but installed El Capitan and switched from FCP7 to FCPX. El Capitan runs flawlessly and FCPX is much faster than FCP7. I don't work with 4K footage, but I'm editing a video with 5 layers on top of each other, including keying and my Mac has no problem playing this. I guess 4K is a different story, so it depends on what you want to do, but for 400€ I wouldn't hesitate.