I asked a local steel fabricator who specializes in trailer frame work and repair what brands of enclosed utility trailers they recommended. They told me Pace or Hallmark were good brands to purchase. They do not sell trailers at all so they have nothing to gain by recommending them.
I've been looking into building a 14 or 16 footer with 7000 lb cargo capacity. I will be purchasing one as soon as I get my shed/workshop built. I hope to retire in the about two to three years and then I'm hoping to go full-time nomad and travel the country.
There are lots of YouTube videos that show people building out their own trailers as well as some full-time nomads giving tours of their trailers and interviewing other people about their trailers and how they built them.
Another source for ideas are those that have built their own RV's using step vans, box trucks or Mercedes and Dodge Ram ProMaster vans. There are a lot of similar ideas with these and the trailers as well.
Bob Wells YouTube channel CheapRVLiving is a good place to look for ideas.