I just looked at a used tractor (in VT, not near you, unfortunately) and the guy was selling a Pronovost dump trailer. He used it to trundle around gravel when he was developing his lot. It will hold 2-3 yards. He pulled it with a 43 horse 4wd CUT. It uses the tractor hydrualic remotes to dump, with two pistons. Tailgate swings down or up I think. It looked like a good unit, he was asking $3K for it maybe 10 years old. Dual axle with smooth tires like light plane might use. Also, we use for our building demolition a Cam Superline dump trailer with its own hydraulic pump that plugs into the regular RV connector with the lights and charges its battery while you're on your way to the landfill. That works real well, we bought it this spring for about $5,300 here in the expensive Northeast, but worth it as we retired an old GMC dump truck. I'd say your main choice is going to be between the systems the trailer uses to power the dumping, more than what kind of tailgate because as you say you can always get out the welder. Good luck.
Jim