If you just want to build a retro AGA WHDLOAD machine running OS3.1 then you only really need a 4MB memory card. A 1200 with the standard 2mb is next to useless unless you like playing with floppy disks.
If you want to run 3.5/3.9 you'll also need an accelerator because that's the only way of adding more than 4mb of FASTRAM and keeping your PCMCIA slot (even an 020 can run 3.9 if you have at least 8mb of FASTRAM).
If you want to run the later (post 93') AGA demos they you'll need at least an 030.
If you want to run some of the later AGA software (esp PC ports) you'll need more grunt than the standard 020 but then again, even an 030 will start to struggle (hello Mr 060...$$$).
Don't bother with an 040 and don't bother with a PPC.
