Kelly Blue Book is around $20k in good conditon. Deduct what it would take to repair the engine well (ie new engine at dealer or good shop). More than 7k for repairs I think. The 6.1 is a high po engine... used examples may be beat if you can find one.

Otherwise, do you know the history of the vehicle? How was it run and cared for? If it was run hard, other parts of the powertrain may be iffy... transmission, T case, axles ect.

It could be a screaming great deal or a money pit.