In my humble opinion just as important as the engine is the fuel system. My engine never let me down in 11 years of use, but it did die 3 times, all from fuel delivery issues between the engine and the tank.
Todays diesel engines are crazy reliable, that does not mean not to take some classes, but don't overlook the fuel system, the transmission or the rest of the drive line. Any of them can bring you to a screeching halt