The use of fiberglass one-piece front and rear end caps have been in play for a number of years. They all but eliminates the potential for most cabover leaks. I would hesitate to buy a class C without this level of construction but then I would not buy any C without a one piece fiberglass roof as well.
As for payload, it usually begins to disappear quickly as C's approach 30' and most over 30' are even more limited. Slides and auto levelers add a lot of weight and will eat into available payload vs C's with neither.
I am not a fan of buying a former rental as they take a lot of abuse by inexperienced operators {never met one yet who even had heard of Tow Haul}.
As always.... Opinions and YMMV.