As Nunzio said, start with a relative rust free frame, maybe something from down south away from the coast. It sounds like you’re looking for a truck, so what a lot of Land Rover Defender folks do is coat their frame, under-body, etc… with Waxoyl. I’ve heard good things about Fluid Film too. Something that’s not a thick, tar mess that’ll hold in moisture. Neither are exorbitantly expensive and should significantly extend the life your truck’s frame and body. You’ll just need to crawl under your truck periodically.