Here is an awesome thread from a member on here about all that needs to/can be done to the 6.0 to make it bulletproof.
Write Up: Bullet-Proofing the 6.0L PSD.
As for buying a 6.0 from an auction, it will be very hard to determine how healthy the engine is without hooking up a monitor to it. When they start the trucks are they cold? If the trucks are cold listen very carefully to how it starts and idles right after start. Also watch very carefully for any smoke. When the trucks are warm the act completely different and can seem like all their problems are gone! Like Magic lol
White Smoke = Coolant (EGR or Head Issue)
Black Smoke = Fuel (Turbo Issue)
Blue Smoke = Oil (Valve Issues)
If you have any questions please feel free to PM me!