the handbook is pretty helpful regarding the regulations
its my understanding that you do not need a cdl even if you are working with the trailer. if that was the case every single person in the trades would have to have a cdl. but the dot number you will need.
as for the trailer i would say get one with brakes on both axles get the controller in your truck. i wouldnt want to pull 7000 without trailer brakes, yea the truck can stop it eventually but what about in an emergency someone cuts you off or slams on their brakes i would take brakes on 6 axles over4 any day