There are 2 different engines for the 04 model year. Earlier versions have a 305hp engine and trucks built after 1/04 are equipped with the 325hp version. In my experience the 305hp engine will provide the best fuel mileage and durability. The 04.5 to 07 325hp engines seem to have more injector problems than earlier 305hp models. The lifespan of both are directly related to how they are maintained. I would highly suggest adding a 2 micron fuel filter setup to the truck to maximize injector life. This is by far the biggest issue with these high pressure common rail engines. The trans should give you good service if the truck remains at stock power levels. Adding a programmer that increases the hp over 50 to 75 hp and you will be testing the limits of the torque converter lock up clutch. Frequent kick down band adjustment is also necessary on these vehicles. Also check the front suspension and steering components for wear. Ball joints and tie rod ends are common to wear out.