If it was only a rad cap, then the owner would have replaced it before he tried to sell the car. If the head gasket is leaking, then water would be in the oil. That can easily be identified by looking at the dipstick for telltale water spots. If the rest of the car is in pristine condition, and you can get it for a song, then the head gasket can be replaced and you will most likely have a nice Mustang, but otherwise, I would walk away from this liar.