Its still possible to fit a HDD inside the case if you move it to another location but the extra heat with the already toasty ACA630 would probably be a bad idea.
An A600 is anything from £20-40 these days + extra £20-30 for a 1mb Upgrade although you can do a piggy back mod. I'm sure someone will burn you a 3.1Rom
ACA630/25 is £136 + P&P, A603 is about £35 and Indi ECS is £85 if you go that way.
I might have a spare A600 coming up that Zetr0 will doing the chip mem mod on (so that he can re-write the guide and take fresh pics for the how-to) once thats done I can let you have it for a resonable price
That would rule out the Indi ECS though as you would need an A603 for that to connect to.
I've got Indi ECS, A603, etc in my A600 - you can see how the all fit together on my project page:
All the best,