The 03-04s have a much better EGR cooler than the 05-07 6.0s do. All the 6.0s have the oil cooler issue. The 05-07s have their own set of issues that the 03-04s do not, such as the STC fitting on the HPOP.
Known problems are that the oil cooler clogs up, takes out the EGR cooler, then blows the headgaskets.
When you look at a used 6.0 take a good look around the degas bottle and make sure that there is no white coolant residue present. If the headgaskets or EGR cooler are bad the truck will puke coolant out of the degas bottle and leave white residue after it dries. When you take the truck for a test drive get it all the way up to operating temp and then get on it to see if it will puke or not.
One of the best tools you could buy to help you search for a used 6.0 is a scangauge II. Hook it up to the truck and monitor the oil temp and coolant temp while you drive at 65mph for at least 15 minutes. If the oil gets 15* hotter than the coolant then the oil cooler is bad. It's an expensive repair so you could either pass on the truck or use the issue to help you negotiate.
2003 Ranger Regular cab 5-Speed 4x4
D30 SAS | KING Coilovers | Deaver f31s | 4.88s | Detroit TrueTrac | Eaton E-Locker | 35" MTZs | 1354m | RE Long Arms | Currie AntiRock