The sheet metal repair that's been done looks like it was done really well. It looks like the unibody itself should be pretty solid. I think the next step now is to have it media blasted, then epoxy-primered before any more surface rust can come along.
I'm missing the part where the seller outlined all that work, or even anything beyond "engine comes with the car."
It might all be there... it might not... what's there may or may not be restorable. I also heard no mention of a transmission - Heck, I don't even see seat platforms or doors for that matter. It's all a crap shoot.
Having said that, if the driveline is all numbers matching, then it might be worth it as a rare, "to be faithfully restored," big-block project car. The average guy looking for a '71 Mach 1 to have fun with need not apply.