I have an old Trek 800 that I want to add a suspension fork. It originally is a rigid body. I don't want to buy a new bike because I don't have the money I figure just adding a decent, cheap fork would be sufficient for trail riding. Is this a good idea and if so what should I be looking for in a fork? Thanks.
Most will tell you that cheap and decent don't exist. That isn't true. For some moderate to easy trail riding must pick up an RST on ebay. They can be found for cheap. You will have to make sure that the steerer sizing is ok, etc.
Someone more qualified can tell you the specifics on the mechanical aspects but I just wanted to chime in and tell you to do it before everyone else tells you not to.