The 5.9L cummins is one of the best motors there is.
I would buy a common rail though (2003-2007). So as you don't have injection pump issues, unless you want to p-pump it or don't mind replacing injection pumps.
but they get great gas mileage, and make TONS of power CHEAP!
600hp and 1400ft/lb isnt hard to accomplish with a cummins