The biggie here is what rust are we dealing with? If it's disassembled, that's probably why. Look at the floor boards, trunk, and cowl panel. That should tell you, along with the frame rails. The fenders are a one year only thing, so it would be nice if they were workable. That's just what I would be looking for now. For 5K, a 70 Challenger should be worth it and then some.
Mike