Depends on how good of maintainance the PO has done on it. Coolant with the proper SCA's level, oil changes 3K to 5K miles, What brand oil is he using? Differential lube change, trans oil and filter change. Is it a 4X4? Will the hubs lock up manual and auto? Has the hubs ever been relubed? Has the lube ever been changed in the transfer case? Steering tire wear. Do the glow plugs operate correctly up to 20 seconds cold, 25-30 degrees, not just on/off for 3-4 seconds. Start the truck when cold, see how it starts/cranks. Is it an automatic "4R100"? Does it shift good, cold or hot? Drive shaft leaks? Brake pads, calipers and rotors. Repairs on diesel's are expensive as you may know.