First, all Spyder's are the same width at the front wheels, the widest part of the Spyder. If an RT is tight, so will the F3, ST or RS.
Enclosed, or not enclosed. Open trailers are lighter for sure. But then they lack the protection from the elements and prying hands. We regularly pull out of the frozen north land, the land of snow and salt. I would only use an enclosed trailer for plans like yours. The enclosed trailer will catch more wind with aerodynamic drag - harder to pull - vs an open trailer. Find one that has a minimum height, which may mean you will hunch over a bit when loading or walking in the trailer.
One thing that hasn't been discussed is classic spring vs torsion suspension. Springs generally add to the overall height of the trailer and arguably, are a more rough ride. Shock absorbers would be nice but I don't know of any makes of trailers that have them in that size range. My pull behind the motorcycle trailer, a Bushtec, has air suspension and shock absorbers, which made for a more enjoyable pull.
The tow vehicle will likely be powered by an internal combustion engine. The efficiency of the tow (fuel mileage) and wear-and-tear on the vehicle will depend on rolling resistance and the wind resistance of the package. Regardless of the engine, it will take a certain amount of horsepower to move the package down the road, or up the hill AND control it going down the hill on the other side. The engine will convert chemical potential into kinetic energy, and there are differences in the efficiencies of engines in that conversion. Torque is the potential that moves the load. I can have high horsepower from a turbocharger, but suffer on the torque side to get the load moving. I would recommend displacement vs turbocharging to get horsepower.
Good luck with your hunt! My current outfit is a 09 F-150, 5.4 liter motor and 6 speed auto, towing package, a 7 X 16 enclosed aluminum trailer, that we load two bikes (two Spyders or a Spyder and H-D bagger). The trailer weight is 1700#, and each of the bikes is ~850#. With gear that we put into the trailer, another benefit of an enclosed trailer, the total weight is 3600#. Fuel mileage is generally in the 10.5 mpg but can suffer dramatically in a headwind (or benefit a bit by a tailwind).
Wayne