Here are some things to consider, do you know if the owner kept up on the maint? Find out if he knows anything about the cooling systems and the SCA additives. By now this truck should have already needed a new water pump or is getting ready to give it up. Has he taken it off road or any severe driving conditions? Ask about the lift kit, did he do it or did someone else, check everything over. Ask about mods and about programmers. Be careful and you may have some haggle room if the guy can't answer alot of these Q's. Then again you may be getting more then you haggled for. The list is endless about what you need to find out, or more correctly stated, what you want to find out. Have this guy go over ever square inch with you if he is "all about" his truck. Replacing parts on these beasts can get expensive quick. Other than that it looks like a good truck.15K isn't a bad number but remember people always leave haggle room.
God luck