There's a few good threads about what to look for with the 6.0L:
Write Up: Bullet-Proofing the 6.0L PSD.
Things to consider about owning a 6.0L
Most of the big ticket items were corrected before the 2007 MY. The oil cooler still fails, and the STC fitting on the HPOP needs to be updated, and really thats about it.
Key things to check is get an Oaisis and a Carfax report, know what got done to the engine at a dealership. If the thing has been studded, ask if the heads were checked when they were off. These are great trucks if taken care of, and problems are corrected early.