Looks in good nick. Regarding underseal, I would tend to clean and patch it rather than just blast it with stone chip. Stone chip is just that, you can't cover rust with it. If its fairly solid I would say cavity sealant is more important.

Rust in the chassis is a real bugger, you need to throw every thing at it, abrasive removal, chemical conversion, zinc paint, epoxy sealant. You usually have the problem that you cannot dip the car after welding.