Run an oasis report on the truck if you have the vin number (somebody at www.thedieselgarage.com
will do it for free for you). That will tell you the service history of the truck and let you know if it has been a problem child or not.
The 03 and early build 04 6.0's had quite a few problems, and I would be nervouse about buying one. I own a 2004 6.0 F-250 and it has been flawless, I am the second owner. It is a late build 04, so all of the problems that the early ones suffered have been taken care of. The only work that has been done to my truck was something with the EGR, and that was before I owned it.
If you can swing it, go for an 05 or newer 6.0 f-250 or F-350...none of the problems of the ealry ones, it has the new coil spring front end intead of the leafs of the previous models, and the looks are a lot better IMO (updated grill, headlights, and interior).