The farm I work at on the side has a 01 dmax my couisin has a chipped 06 dmax drive them both and love them I have also drove a cummins and am a dodge guy. I would say get a 06 07 5.9 cummins better torque, and MPG is great. Guys on the dodgeforum hav seen 22 maybe higher mpg driving around empty and see around 18 pulling.
1997 Ram 1500
2005 Ram 2500 HO Cummins
1997 Wells Cargo 6x12 enclosed trailer
2013 x739 4x4 with 54in HC deck
2010 Toro Grandstand 48in