Well, you'd have to see the rig in person I'd think and get up on that roof yourself or hire someone to inspect it for you. Then all you can do is take it from that assessment.
Other's have pointed out some good things to consider too, so you would be taking on a big project that could only get deeper once you open it up..
Hard to say. Guess it depends on how much you want it and how much more $$ you feel comfortable investing to repair it...
You could just low ball the offer and see what happens... But, then the seller might accept it! So be prepared for that and only offer if you are serious about it.
Good luck! Mitch
