well if it would be possible with an "up-to-date" A500 mobo replacement, with.. ahem... "modern" features, then , that would be the choice for me..
There are so many old A500 out there, where the chips works perfectly. And with the Zorro slot its very easy to expand, more so than the A1200.
Also, the casing of the A500 have more room for extras, like build in CD-ROM, and so on... Give it a PCMCIA slot, and you could, in theory add just about anything..
Build in ide or sata controller.... would be nice..