To add to Gregs Ford story- The reason the 7.3 diesels hold their value so well is that nobody wants the 6.0. It was a nightmare, and Ford still can't come up with a head gasket that will stay in it. The 6.4 is better, but mileage sucks due to the particulate trap. That can be solved, but not legally in a polution controlled jurisdiction.

The gas engines don't require a hamfisted plug change, although that does help. Early engine have 3 threads in an alumunum head to hold the plugs. Bad design. There is a commonly available Timesert kit to repair the heads, but its a pain. The later 3 valve heads use more threads, but have a two part spark plug that breaks when you remove them unless you do a two day decarbonize/penetrating oil/wiggle program. Again, there is a commonly available puller kit to remove the ceramic and unthreaded spark plug extension. Quick solution is to sell the truck before you have to change the plugs, so watch out for that when you are buying.

As to transmissions, add a trans temp gauge, and drive by it. Temp kills.