As the previous owner of a 1977 MGB roadster, I would have to say that if there is "bubbling" in the paint and you don't want to spend money on body repairs, WALK AWAY! At $5k, I would expect it to not have any rust. There are several nice examples here in Tulsa, OK that have zero rust and are in the under $4k range. Surely there are some in your area that don't have any rust and are in that price range.