I had a long shopping list for mine: rear engine, Cummins 8.3 or bigger, air ride, high roof, storage bays, air conditioning, and can't remember what else. Truth be told, I don't know whether what I wanted could even be ordered from the factory, much less found on the used market!
Finally I found one with the Cummins 8.3 in the rear and resolved I'd have to fill in the rest if I continued to feel strongly about those. So far I've raised the roof and am in process building storage bays. It was nice to know what I wanted, but in the end the #3 requirement after pusher style and Cummins was "sooner rather than later."