I would keep looking if I were you. I bought a 1991 F-250 diesel 4x4 last fall with only 135K on it, and right away had to have an injector pump, injectors, glow plugs and a controller put on it. I know that Chevy has just had the pump done, but my total bill was about $2400 for parts and labor. The lesson is that diesels are very expensive to work on, and without knowing how many people owned it and how it was cared for, you are taking a big risk. The motor may go to 300K before needing internal work, but check out how much they want for a long block before you commit to this truck. Even if the motor and tranny hold up, that is almost a quarter of a million miles on the suspension, brakes, electronics, climate control system and body. Even a well cared for truck will be worn by then, and diesels take more care than gas engines. Also, gas mileage varies widely by vehicle. I have never broken 15 mpg in my truck, with 3.54 gears, an automatic, and driving 75 with cruise set. Others get near 20 on the highway with the same setup, and there is no explanation for the difference. If you go for it, take it to a shop and have them check it out. (Fuel pressure, glow plug resistance, compression, etc.) Check the rear end too, if you don't tow you don't need 4.11 gears, and they will hurt your mileage. Good luck either way, but you are better keeping a headache you know that buying an unfamiliar one. The Ford/International engine or Dodge/Cummins straight six are far better powerplants for longevity and power, even with no turbo.

PS Stick with a manual tranny if you can, diesels chew up automatics with frightening speed if you don't drive like your grandmother, and they are hideously expensive to replace. (This goes for Chevy, Ford, and Dodge.)