I think it a great way to go, I'm surprised more people don't do it. 5x8 is a great size, I had one and took it off road a bit. You want to get a good quality trailer like Wells Cargo, make sure the sidewall framing is fairly close together, 16"oc is great, and that it has 3/8" ply screwed well. The weakest point imo of using a cargo trailer ofroad is the rear door(s) usually takes up the whole rear of the trailer thereby removing the lateral structural element needed to keep the trailer from racking. If I were to modify a 5x8 for offroad use, I would weld 1' wide shear walls from floor to roof on either side.
Add some long soft springs, and shocks, amulti axis tongue/hitch and you're good to go.
My .02
Kye