I don't think you'l get better results out of cheap sunnies. The ones on my Sierra (15x7, $62 from tyre power) aren't even 18 months old yet and there's noticeable rust breaking through the paint. I don't think I've ever even cleaned them and it's usually not garaged, still not a great result though. They're white so it's a lot more noticeable than black. I drove a lot of sand but I never had it on the beach IIRC. Dunes stretch for KMs before they even get within sight of the beach here.
Rust guard has a 5 year guarantee against rust if you prep them properly, that's about what I'd expect out of some powder coated ones too.
Yeah, you can swap tyres between wheels. It takes a real monkey to stuff the bead up. If I was you I'd get some new tyres and wheels and sell your old ones. Should be able to get a decent amount for them.
