Lots of different answers and all valid in one way or another. I agree that you should eBay the NOS A2000 as that will get you the most money to expand any Amiga that you choose to "Build Up".
The cheapest and easiest way to go would be to put your A1200 into a tower kit with a Zorro board and perhaps a Mediator PCI expansion. With the PCI slots you can add the fastest Graphics and Sound cards and a cheap network card.
Find an 060 or PPC accelerator for it and you should be all set.
WHDLoad will solve all compatibility problems.
The A3000T and A4000T are other good suggestions, but will cost you more and won't give you much more in the end.
