Purchasing a car from a private seller. Nightmare.

That sounds promising. The frame rail(s) most certainly can be repaired. Many members of this very forum have done it. It requires a lot of drilling and cutting and general metal fab, but it can be repaired. That is the only way I would be happy with it is if the frame rails were replaced.....and I would make him look at all of them, front and rear. This is likely to be your first and last shot at it. He's probably not going to let you just keep coming back. Good luck!