The 6.0's can be great engines AS LONG AS they have been taken care of. If it were me, The first thing I would do is pop the hood and look for the white residue on and around the degas bottle. Then crank it up, run it around till' its up to operating temp, then give it a little hell. Do not be hard on the throttle until its up to operating temp, but don't be afraid to make some hard runs either. This (especially during the hot summer) will allow pressure to build in the cooling system and push out the degas bottle if a head gasket is compromised. Also pay attention to any hard starts or rough idle. If the buyer will allow, get an oil sample and overnight it to Blackstone (The kits are free and they send em' to you quick if you call and explain the situation.BTW, great company to deal with IMHO). The analysis will tell you of most any problem the truck could have with the engine (abnormal wear metals, coolant or fuel in oil, etc.). Another thing to be weary of is a K&N air filter, they have been known to allow dirt past the filter and into the intake. Check the brakes, under carriage, axles, and look closely at the bottom of the motor for leaks. Auto Ingenuity software will come in really handy to check your oil vs. coolant temps. More than 15* difference means you got a clogged oil cooler. The scangauge II will also give you some reliable temps if the AI isn't available. Usually if someone took good care of the interior, paint and kept good tires and brakes, then they took the time to change the oil regularly and stay on top of maintenance. Maintenance is key on these trucks.