If you buy 7.3 psd look close at oil pan! if there is any scaling at all, walk away. I am on my 3rd oil pan on my 2001 PSD, and two of my friends are on there third! Also they are famous for glow plug wiring circuit problems, expensive transmissions, and there is a cup in the head around the injectors that has broke in two of mine, the only way you know its bad is diesel fuel starts spitting out the radiator overflow, because the fuel is under higher pressure and the cup seperates the injector from the coolant!!
Go with cummins!!!! I have had three all with over 200k and the only thing to watch for is fuel pressure( install a guage) which is a worry on most diesels!!
You don't here to much about PSD swaps!!! there is reason people use cummins!!!