The high dollar items that seem to hurt a guy that I would want covered:
Headgasket
Turbo (esp on Cummins with VVG)
Air to Air
Injectors
Trans (aux sections tend to go first as guys are ignorant of how driveline angles can tear one up - if you feel a vibe - get it checked!)
Diffs (rare-ish but they do happen)
Had a c-12 come in - clatter noise - found a 50 cent piece of the #3 cylinder liner in the pan aloing with a valve - piston in the bore sideways - only choice was junk the engine - total cost to replace with reman - was pushing 30k - engine warranties are worth every penny.
Water pumps cost money - hell everything on a truck does but I would concern myself with driveline the most when it comes to condition of the truck.