Look at the degas bottle under the hood. If there is any residue from any coolant usually means that the head gaskets are blown. Like I said before I am not trying to scare you away or anything but with that many miles with the DPF still on it and if the maintenance wasn't done very well I would be worried about the motor going out on it. I think after you get a new long block, the cab pulled and the new motor installed you are talking about a $10k+ job. Obviously I am pointing out worst case scenario but you will have to decide if the risk is worth the reward. Based on KBB with the normal options on that truck it is worth around 17K. Usually when folks sell their car they use this as a base line. My question would be why is he only asking 13,700? Is he looking to get rid of a problem? I am a skeptical person is my issue and I like to buy new so my view point is biased.