A guy could write a 5 page essay to cover that question. You'll want to narrow that down to either a price or age range your bro is interested in. Engine wise, generally the most reliable ones are the 12 valve engines, which are mechanical engines in that they have no computer controls. Those were put in dodge trucks until 1998. In terms of transmissions, the 90's/2000's 4 speed auto is generally the worst, not holding up well to the output of the cummins. As the trucks got newer, engine size, power output and complexity all went up while fuel mileage generally went down.