It just seems really unlikely to me that the frame is bent....... Obviously there's a reason for you to think so, but it's not readily apparent from the pictures. If the forks didn't bend, why would the frame?
If the owner is telling you this, hem and haw, low ball them and buy the bike. That thing's more than a 600cc machine, right? Is it a ZX 7? I don't recall which models ran the air through the sides of the frame rails like that...... but I'd think it's at least a 750. The muffler volume is big enough to quell a liter bike....
Dump the gas, put fresh stuff in there with a bunch of carb cleaner in it, flush the brakes, flush the fork oil, and see where you are really at.......
A good motor and wheels.... you won't lose any money on it either way. It's probably worth at least as much in parts as it is in the whole.