Is there an issue with the 6.0 diesels in 04? I remember reading something about them but I can't find it.

The second turbo likes to blow up and I believe something about the fuel filter, plastic in there breaking or melting. I can't remember. Hopefully someone else will chime in or, you can head over to thedieselstop.com and search their ford forum

'03 was the first year. They really didn't iron everything out until 05/06. '04s are still questionable.

Biggest questions to ask are mileage, how has the truck been used, maintained, and what (documented) maintenance has been done?