CTD is a 500K+ mile motor, and even more if taken care of. Head gasket for the 6.7 is weaker than the 5.9, due to higher boost pressures with the variable vane turbo. It was common on the early 6.7, to get misdiagnosed due to an EGR cooler leaking. At 1200 miles away and high mileage, to me, would be price. Have the interior completely detailed and cleaned, if it were me, I'd pull all the seats and go through them as well. I have the oppisite effect, I have a low mileage 07 C&C 6.7 that has minor problems from not being driven enough. If a CTD has high highway miles, then you could consider a 15 to 20% less mileage then the odometer shows, as far as the wear factor. As mentioned prior, an engine run at designed temps is beneficial to the engine, espeacially the CTD. Good luck with your negotiations.