Yep, looks like a traditional midwest vehicle that the owner took GMs word for that "no additional rust proofing is necessary. "
You have a couple options: pressure wash the heck out of it with a turbo tip and blow off any and all loose coating and rust. Let dry and then apply rust converter followed by some sort of sealant (paint, truck bed coating, etc). Wait for that to cure and then apply fluid film liberally all over everything, inside rocker panels, fenders, doors, etc.
Or: just take it to zeebart and have them do their rust killing process followed by a good rust proofing.
These are the only ways to make it last longer in the midwest. Believe me, I live here too, and have worked on grungy cars for too many years. I now buy western or Southern vehicles, rust proof them myself, drive them, and then enjoy a clean vehicle for years to come while others of the same era fall apart.
I'd say your vehicle is salvageable at this point, just don't skimp on doing it right. And, be sure to do annual touch-ups as needed. I have to every year before salt hits the road just to stay ahead of it.