Either hitch receiver would work just fine. The Drawtite version should clear oversized tires on the stock spare carrier, while the Curt may be limited to stock tires. Also the drawtite can carry 1500 pounds of tongue weight.
As to the Turtle Top... All those buses are generally built to order. Any of the 158" wheelbase floorplans could be built on an E450 instead of an E350. Keep in mind the E350 dually also has about 2500 pounds more carrying capacity than a single rear wheel E350.
The other company that makes a similar bus is StarCraft, called the StarQuest
PS - There is one member on the Class C forum with an E450 Turtle Top. His was custom built with a mild camper build (bathroom, fridge and couches etc) to use for weekend trips and tailgating.
