The 7.3 powerstroke is an absolute workhorse of an engine, even mildly well taken care of, you can expect to get 400k miles out of one without having to do anything more major than replace the injectors.
The 4R100 Trannies that the 7.3 got after 96 are similarly hardy (I have some issues with the sturdiness of the E4ODs).
Moreover, the engine itself is pretty easy to work on. The problem comes from it being crammed into a E-x50 chassis. On the vans, everything is so tight that access is a constant problem, I was forever taking it to a mechanic to do things I absolutely could do if it were in a F series, but just didn't feel like dealing with on the E series (I've got big hands.).
If you build on a type I, I'd strongly recommend the 7.3 powerstroke, if you've got your heart set on a type II or III...well, I'd probably still recommend the powerstroke, because I doubt any of the others are better, but I'd at least physically look at the others first.