Hi Dr John,
First question I would ask is what are you going to be shooting with it? Paid work or hobby?
The Sachtler Ace range is excellent but pricey if you're not professional. Still, a good quality tripod is essential and a good investment. Miller, Manfrotto, Libec are all fine, too (I have one of each). Because the camera is so light you don't need to spend a fortune, as long as you have a decent fluid head and legs that don't twist when you pan.
A monopod is very handy, particularly with a nice lightweight camera like the GH3. Mine's a Manfrotto with a video tilt head on it and I have Manfrotto quick release plates on all my other support gear so they're all standard across all my cameras/tripods etc.
If you get an iMac at least all your bases are covered for all the NLEs out there. A PC will exclude either flavour of FCP.
I disagree with Axel's "old-fashioned" comment. Premiere Pro 6 is anything but old fashioned and will do whatever you want it to.
I can't speak for FCP X as I haven't used it. Don't like it. But that's just my opinion because I regularly cut on Avid, Adobe & FCP7, which all have a similar feel. FCP X is completely different. Having said that, if you have little knowledge of modern NLE's and you get an iMac, I would try FCP X first. Get the trial version off the app store. If you don't like it, try the trial of Premiere from Adobe.
Unless you're planning on becoming a colourist, the built in colour correction tools in Premiere, Avid, and Final Cut are more than adequate.You can also buy plugins to get pre-built "looks". Working with 4.2.0 colour doesn't lend itself to too much post manipulation anyway.
Save the rest of your money for lenses :)
Cheers,
MW