sounds liek a lose ground connection or some other electrical problem and a transmission issue...
if the truck was never driveable well to you at least, i am willing to bet your tranny's soft parts are shot and it needs a rebuild. it is very common for people to sell a vehcile at that point... usually they will run a little bit when cold but wont move at all when warm. and the fact that reverse works fine and you have no forward gears....
