If you get an enclosed trailer, you won't have weather or flying debris damage to your inventory. It will be much less accessible to would-be thieves seeking targets of opportunity, may even reduce insurance costs for theft/damage to inventory. It might even be more aerodynamic and tow easier with less wind resistance, ergo, better fuel mileage. As to CDL and DOT requirements, some of the regs are about as clear as mud. I drive for a living, my truck is DOT inspected, I have a class B , and so far it has been all I need. I also stay under 26k. I try to stay in compliance, and I keep my logbook up to date. Also, I'm leased to a DOT carrier and they help keep me updated on changes, etc. Best of luck, and if something doesn't make sense to you, by all means, get a DOT officer to explain it to you before you get pulled over. DOT has a website, as was mentioned earlier in this thread. Most of it isn't very clear or helpful. Which is why I recommend getting real world advice from a DOT officer, instead of listening to a bunch of opinions from people who have nothing to lose if they are wrong about YOUR license and YOUR business.
cover yer a$$
Dave