I go to Expo Portal quite a bit too. Quite a few people convert utility trailers that way. Seems it would be quite a bit more economical as a decent utility trailer can be had for a couple hundred. Even with the floor needing replaced, he most likely could have sold that one, purchased a good utility trailer and had some left over for materials for the build. Just my thoughts. Either way, the outcome is nice.
I plan to start one soon once things start to warm up a bit.