The main thing to look for is flat spotting on the under side of the chassis. The components may look great, but if the frame tubes are flat spotted, the frame is gone. A small amount of flat spotting is normal and not of concern. But if the flat spotting is more that 3/8″ wide, I would steer clear. The main places to look is under the waist and under the rear bearing hangers.
In my experience, I usually see that you have to put so much money in a used one to replace components to make it reliable, you would be better off buying new. Tht is not to say there are not good deals out there. Some equipment is very well cared for and if the guy is getting out or changing classes, you may well get a ton of spares for free.