As far as longevity based on the fact it is a diesel, my buddys V-10 just passed over 450,000 miles then a cylinder failed, another friend just passed 275,000 and he replaced an alternator.
Both are crew cab pick ups but the same motor. Read the thread about those that have purchased from rental companies and their experiences. As far as mileage a class C gets 8 MPG. Sometimes going down a hill I get 11. It has a 55 gallon fuel tank for a reason.
Rent a unit ,go somewhere and see if its to your liking and go from there.