Welcome back. I just went through an A120 build a few months ago so maybe I can help. I bought a 1200 system from Amigakit and they did a wonderful job configuring it for me. Really top notch.
I was about to buy the E-matrix 1230 card from them but at the last minute somebody had a Blizzard 1260 card so I bought that instead. I was looking for the ultimate accelerator but it came with a price. I did research the E-matrix card and it seemed to be a good choice.
I wouldn't bother with a CDROM drive. What I really found to be most valuable was the 1) Indivision AGA for hooking up to VGA monitors, 2) PCMCIA CF-Card reader to swap files between my PC and the Amiga. 3) Internal CF-card hard drive 4) Cocolino PS2 mouse adapter so I can use an optical mouse.
Other things that I added which haven't been as useful are: USB adapter, PCMCIA wireless ethernet (not used because doesn't support WPA encryption). I eventually picked up a 3com Etherlink III off ebay for $1.
I don't have any experience with RTG (retargetable graphics) cards or the mediator as I left my 1200 in the original case. It seems a shame to tower-ize a new 1200. Maybe you should get a 4000T instead?
Good luck with your build out!
