[cabmandone]

The transmission behind that beast is a known issue.
Other than the transmission not many big issues if you're buying a non rust belt rig. I have 225K on my 1999 4x4 crew cab. I have done the 6.0 transmission cooler upgrade, pillar gauge cluster with EGT, Trans temp and Boost, 6637 air filter mod and installed a larger power steering cooler as well. I have had a few of the common problems. The wire harness under the valve covers for the injectors had loose clips which caused the truck not to fire. 50 cent mod fixed this problem. I had the rusted out oil pan issue as well. This one is a pain since the engine does need to come out to fix this problem. A big one but again it's a rust belt issue is rusted brake lines that burst. Yeah... I had that one happen while pulling a skid steer. If you're looking at 4x4 and it has Electronic Shift On the Fly check to make sure it works. The system operates on vacuum. There is an O ring in the knuckle that can fail which keeps ESOF from working. Also rotate the manual lockers for the hubs. These have a tendency to freeze up when they aren't used. Most of these suffer from "road wander" which is tough to fix without spending some money on upgrading the steering box and installing a steering dampener.
A great resource for the 7.3 SD Truck is Ford Truck Enthusiasts. It's sort of the Pelican of the Ford Truck world.

I saw someone mentioned fuel mileage. I get about 17 mpg with my 4x4 on the highway but the key is keeping it under 2000 RPM's. Pulling a 14K equipment trailer with a 10K skid steer I'd get about 8 mpg but I ignore that 2000 rpm rule. Putting around town/mixed driving I get around 11 but I seldom check that one.

A non rust belt common spot for rust is the truck bed at the wheel wells. Ford had a terrible design that cause rust regardless where the vehicle is driven.

If you want a solid work truck I'm selling my 99 sadly. I bought a newer truck for tax purposes and don't need two. $7500 and no, I wouldn't hesitate to take it on a long trip.