The last truck I owned was a 2002 Ford F150, extra-cab 2x4. 5.4L triton with the 5r70w transmission. Like I stated, they will tow just fine, but he should not be expecting to get there fast. The point you make about OD is EXTREMELY important and true. I should have deleted that part from the quoted text, sorry about that. [edit: just removed it from quoted text]
Have you priced out a set of injectors for a 7.3?
Have you priced out a high pressure oil pump?
Have you priced out a turbocharger?
Have you priced out an IDM?
Have you priced out a complete engine? ($12k BTW)
Yes, I worked on diesels and the triton engines when I worked at the dealership. The diesels will last a long time if they are properly taken care of, as will the gasoline engine. The parts listed above are the most common things I worked on while I was in the shop, and by no means were any of those repairs inexpensive. Normal wear and tear items are near the same ammount as a gasoline vehicle, but still, in the long run (with the idea of keeping this vehicle for quite some time) the diesel will cost more.
If you do end up getting a powerstroke, make sure you stay away from any truck that has/has had an oiled aftermarket air filter put on/dropped in the filter box. Those things kept me extremely busy replacing dusted engines at the customers cost-