the only cold start issues for a DMAX that's been properly maintained is gelled fuel. unless you don't plug it in and use thick oil at -40f the 06 uses glowplugs and grid heaters, and has a factory programmed elevated idle (with turbo brake), takes mine about 5min or so to warm enough to start blowing heat from a start in single digit temps
no intake or exhaust needed for a programmer to do what you want it to do it's all able to support close to 500hp in stock form, but the allison won't like it.
the dodge preference for towing comes from the ever present overly stiff like a rock suspension. great if all you do is tow. but other than that the DMAX and cummins are pretty comparable in power and torque curves.
which tows better comes down to gearing, a 4.10 geared cummins will pull a lot harder than the 3.73 or 3.42 geared dmax. from experience being on both sides of that fence, the dmax trucks pulling bigger loads always flew right by me on grades with huge 5ers etc.. so thats what I bought