What kind of seats? Gaucho fold-down sofa beds? Dinette benches? Leg-o-matic folding chairs?
Gauchos can often be repaired/adjusted and have new upholstery put on. I think most of the time unless you had the interior from another trailer, you'd just build a dinette up from raw stock. And Leg-o-matic chairs are often found on eBay, though not often with attractive prices.
As to polishing, I have only hearsay to offer... having a painted trailer I don't have to think seriously about polishing.
My impression is that it's in the neighborhood of $125-$150 per linear foot to have someone else polish it. I know one guy who's an Airstream service professional who, having done some polishing, has decided it's not worth it for him to offer as a service even at the going rates, but I hear there are people who do it for fun and profit.
Oh, and welcome to the Forums... are you looking to upgrade a trailer that's usable? Or do you have a big project on your hands before you're ready to go camping? There are quite a few vintage trailers around the metroplex, and there are a few professionals scattered around who do good work on them.