Rob what have you seen for deals on 16-17 ft trailers? The one for our big duck boat is too small and that building just collapsed on it so I am guessing it is toast so I am in the market as well. I have been keeping my eye on CL but maybe new is the way to go.
As for rust we just wire brush ours down and add new rustoleum every year. By the end of the season most of it is chipped off from gravel but it protects it enough to keep the rust from ruining it. I am sure bed liner would work a lot better but for $10 in rustoleum it is tough to beat the price as long as you don't care what it looks like.