In terms of rust, Southwestern US. Be sure what you’re looking at didn’t come from the rust belt (check the title history). There may be good deals in here in New Mexico, Arizona, or west Texas. You may also check Southern California, but I don’t know the market there as much.
Places with high humidity and close to the ocean, not necessarily snow country, can also be hard on vehicles.
We get snow here in New Mexico and they salt the roads (Albuquerque is a mile above sea level), but it’s so dry that rust isn’t much of an issue. Dry climate is what you want, but in places where it’s too hot like southern Arizona the interior, wiring, and anything made of rubber or plastic might be shot because of the excessive heat.
You want dry climate, but not too hot.