Wow... that's a lot of questions.
For the ones I know the answers to...
I discovered amigakit.com
Amigakit seems to be reputable vendor
Should I go with their beefed up power supply or pick one up from ebay?
The stock A1200 power supply is wimpy. A beefed up one is helpful, especially for any expansions (030, internal hard drive, pcmcia cards, etc.) Now, you can buy a newer custom supply, or just use an A500 power supply. (The A500/1200 power supplies are pin-compatible, and the A500 ones had a decent output.) That choice is yours. Personally I just use an old A500 supply on my retro A1200. It works for the 030 and CF/HD just fine.
What is the video situation on the 1200?
The 1200 has the 23-pin RGB port, plus color composite and color RF built in. (No need for the A520.) There are cheap DB23->HD19 converters. These will allow you to plug in a VGA monitor. The problem is, you'll need a scandoubler or a monitor that can accept a 15khz Horizontal sync to be able to display the NTSC and PAL screens.
Is the composite out on the 1200 still black and white like the 500 or is it color?
What's the rca connector next to the power connector?
See above. There's a color composite output as well as a color RF output. :-)
Which os version should I get? 3.1 or 3.9?
Which do you want? 3.1 is probably more suited for a retro setup. 3.9 has some more things built in, but needs some more hardware to use it fully...
Is the mouse a standard 9-pin serial mouse or is it proprietary?
Amiga mouse ONLY. Don't even think of plugging in a serial, unless you want to cause some damage. There are PS2->Amiga mouse adaptors available, though.
Should I get the memory upgrade from amigakit or will I not notice the difference?
The A1200 LIVES for Fast RAM! It's quite a dog without it. Even better if you can get a 030 card and fast ram. Really makes the little bugger a retro-speed demon. Just the Fast RAM alone makes a big difference, though.
