Personally I wouldn't (and didn't when I bought my used diesel truck) even consider a Ford 6.0 (unless you want to spend a lot of money to "bullet-proof" it). Yes, of course, some have had good luck with the 6.0, but there are many many who did not and spent a lot of money to make it as it should have been when it left the factory, but was not. Google Ford 6.0 problems and read about it to your heart's content.
In the older diesel trucks, the most desirable (read reliable) ones are the Ram 5.9, the Ford 7.3, and the 2006 Chev / GMC Duramax LBZ. Good luck!