The 6.0 is a disaster and there is no such thing as a "bulletproof kit" for them either. None of it works. There's a reason they sell so cheap and seem to be in such good condition. I've had a couple of friends get roped into the 6.0 trucks that had 150k miles, "bulletproof kits", and sold for like $14k. Tried to warn them, but they said it's such a good deal. Within a year they needed head gaskets again.
The 6.4 that replaced the 6.0 is not much better.
The 6.7 seems to be a good engine.