I use to be a dedicated Ford guy. I've owned a dozen Ford pickups over the years and had pretty good luck with them until I got the 6.0. It ran great for the first 100,000 miles, but I put a tuner on it and ended up blowing a head gasket. I spent $8000 getting repairs and upgrades. My big complaint was the way the Ford garages responded. They strung me along until the truck was out of warranty and then offered to fix it for several thousand. They were aware of the problems and knew what was wrong and didn't do anything about it.
Blown head gaskets and bad EGR coolers are pretty common in the 6.0. If you test drive it, stomp on the accelerator and look for white smoke. If you see it, walk away. It indicates coolant is getting into the combustion chambers and it's going to be expensive to repair.
If you decide to buy the truck, I recommend not putting aftermarket tuners and chips on it. The head studs can't take the additional combustion pressure and will stretch and cause problems in the future.