I am new to Diesel engines and am looking to but a nice clean 2004 GMC Sierra 4x4 2500 HD with the LB7. It has 259k miles. No leaks and mechanic checked injector pressures and all were good. Clean inside and out. What's the mileage duramax's are going these days? Any suggestions b4 spending 9500 dollars. How much life is left in this engine? Truck drives great.