@kidkoala
making his own SCSI expansion ? when did I say that ? Why do you think I'm against sidecar expansions ? That's what I'm suggesting. But I claim IDE ones cost higher. SCSI sidecars are cheaper. Anyway, not that it really matters ...
That's a very nice GVP HD8+. But it's also a SCSI unit

SCSI sidecar expansions by far out number IDE sidecars, that's why they're cheaper. Also, you'll see that this HD8+ won't go anything less than 60-70$ since it's a nice boxed unit. it may go well beyond 100$.
@circlip
a a600 motherboard is way smaller than a500 mother board. You can easily fit it, however you'll need to secure it somehow, as screwhole locations would probably be different. Another problem is the keyboard. You can't use the a500 keyboard directly. You'll need to make yourself an adapter which clips on the keyboard controller chip of a600. you'll need a pllc socket and do some soldering 8-)
a1200 mobo is also smaller than the a500 mobo. You can put it in a pc tower case but you'll have to a lot of hacking.