Good luck on your restoration. I'm doing the same and have considered many of the same questions.
Regarding flooring, I looked at some plastic panels (Codemco, for example). Weights were slightly to a lot more than plywood, depending on material and amount of air incorporation. The lightest was a foam polyethylene, but mechanical properties (Elastic Modulus, etc) were not as good as plywood. I think the best alternative might be end-grain balsa with epoxy resin. There are vendors who will custom make these panels for you if cost is no object. I'm going to stick with high quality plywood with epoxy edge sealant.
For insulation, there are lots of applicators who can spray a closed cell foam. This is a very attractive option. I understand that the quality (waterproofness) of the foam depends a lot on the skill and training of the operator.
I plan to install closed cell foam sheets, where the sheets were made in a controlled factory environment. I think the integrity of the closed cells is probably a lot better. These are polyethylene or polyprop or urethane. I would stay away from the common foam building products that will probably disintegrate with the amount of shaking that a trailer gets.
Here's a site that has info on closed cell foams Polyolefin foams