As I said earlier, most times it's easier to buy all-new equipment with Apple products. This would solve all your hardware problems in one go, as far as I can see. If you could use an iMac for your imagesetter machine, I'd go for that - find a Rev. A and whack in more RAM, maybe a mezzanine card for any special connections, etc. Or one of the new base model iMacs with 64MB RAM - not a bad buy at all. And if your imagesetter can be accessed through Ethernet, you're laughing.
Decisions are a part of life, and buying has always been a matter of caveat emptor.
The best advice I can give you is to hunt for someone with a proven track record in selling computers, and pay them to solve your problems for you. And when you're happy, make sure all of your friends know about why.