Like others have said, it is over priced. That being said it sounds like you're sold on it already, so if it's what you like go for it. Guy might go down some anyway, might be why he priced it high.
As far as durability, yes, the T19 is the sturdier transmission, you sacrifice some longevity for the OD.
Although I won't go with a single cab, super-cabs for me only, you don't want a crew dually unless you have a use for it. Mileage sucks, parking is impossible, registration is killer.
People worry too much about cavitation, if it was the big concern it's blown out to be we wouldn't be driving these trucks nearly 30 years later.
And worrying doesn't accomplish anything, you could die tomorrow, but worrying about it isn't going to forestall it any.