the A4000 will give you the most bang for bucks...
I agree with that, apart from the 3000 has SCSI, a built in ScanDoubler and much better case (albeit minus the 5.25" bay). The 4000 has AGA, but it also has crappy IDE and no SD, and the cases are bloody horrible. Take your pick - 4000s are usually easier to find.
pimping an a2000 is not worth it!
That bit's a load of bollocks though (IMO, obviously). I have a 2000 with a 32MB 040 card, CV3D, GVP SCSI (1GB HDD and CD-ROM) and a XSurf and it makes a REALLY nice 3.9 machine. They are a bit more quirky than 4000s and obviously don't have the extras the 3000 and 4000 have, but with all the Zorro slots available and the really solid case it makes a really nice big-box in it's own right. Don't be so quick to write it off. Also 1500s and 2000s are a *lot* cheaper and easier to find.