[Red88Carrera]

12k is about right for that truck. The 7.3L diesel is VERY desirable for its durability and fuel mileage. The grew cab is great too. If all the service was performed at the dealer, get an OASIS report on the truck and find out it's history. It will tell you any time that truck has been to a Ford dealer, even if it was to change a light bulb. You can expect about 18-20mpg highway and about 16mpg towing a moderate load. 136000 miles is just past the break-in period on that motor. With the proper maintenance, a 7.3L will go 300k-400k miles before an overhaul.
I just went from a 2005 F250 with the 5.4L gasser to a 2005 F350 King Ranch dually diesel. Paid just shy of 25k. I looked around for a while and found that used diesels of any kind don't stay on the lot very long. Most were gone within 7 days. If it were me, I'd offer 12k. If he holds out, he'll probably get 14k for it. Remember...CASH talks!