Old computers are actually quite easy to restore if your only enemies are dirt and adhesive stickers. You run into problems if there are broken pieces of plastic, scratches, and yellowed plastics. Even keyboards can be made to work like new again if you have lots of time to spend on them.
However, as cheap as most of them are these days, there isn't any point in wasting the time.. I have bought several machines off of ebay for $10 that were in like-new condition. I even bought a Commodore 64c off of ebay for $9.99 that had the original box and manuals, etc. It looked just like new.
