I have a similar enclosed trailer. Since I got it used (and dirty), I put some work into it before it went into the fleet. First, I cleaned and painted the inside. I then put a new floor of bamboo over the original deck. I also welded a new steel angle piece by the door so that rain and such stay out. It still needs exterior paint and I'll get to that some day. The floors in these enclosed trailers are fairly good in stock form, the supporting frame allows for a lot of weight.
New floor:
New steel edge trim: