I recently built a bike for my brother using old parts. It's a '94 Trek 930 frame, Rock Shox Quadra 10s (2 whole inches of travel!
), an old STX front derailleur, Exage cranks, LX rear derailleur, and so on and so forth. It turned out quite nicely, and I cleaned out my closet pretty well.
New items he had to buy:
Front wheel
Cables
Chain
Seat collar
Innertubes
Front brakes
He also bought a cheap Cateye headlight and taillight, as he rides only at night. He spent a total of about $160. It will need new shifters eventually. I used some ancient Altus rapidfire plus shifters and one of them just kind of fell apart.
I put it back together and it works great for now, but I can't see it being very durable.
The reason he chose to go this route is because he couldn't find any new, fairly affordable bikes that he liked, and the Trek frame was in great shape and reasonably light. I say if you
really like the frame and fork, then go for it. Otherwise I'd buy a new bike.