International and Ford designed the 7.3L to last a mean of 250,000 miles under average maintenance. But average maintenance means horrible maintenance by my definition. So well-maintained engines can expect to last over 400,000 miles. With commercial usage so they rack up lots of miles every year, you can expect over 500,000 miles before the first overhaul. But for Average Joe that puts on 20,000 miles per year on the truck, then about 20 years or 400,000 miles is about all you can expect, even with outstanding maintenance. Mine's over 10 years old now, with less than 200,000 miles, so I'm expecting it to last another 10 years with continued excellent maintenance and less than 20,000 miles per year.
I'm looking at at truck with 278k.
Miles are not nearly as important as how those miles got there and the maintenance the engine received over those miles. If my truck had 278K, I would expect it to last at least another 100k. But if Joe down the street had 278k on his, I'd expect it to go into the dumper any day now.
So if the owner has maintained meticulous records that show every oil change, every fuel filter change, every coolant change, then I wouldn't worry so much about the miles on the engine.
Of course, other than the engine the diesel Ford are just another truck. So the rest of the truck might not last as long as the engine. Maintenance can help there too, but the 4R100 automagic tranny and torque converter is probably not going to last as long as the engine, so knowing when it was last replaced can have an impact on your decision as to wehther to buy the truck and for how much.
