Yeah I'd like a Cat mechanical ideally, too.
The new DEF systems do cause trouble, but to the best of my knowledge Macks give 99% of the problems. As soon as you get a genuine cold snap up there (-40c and below), there are new Macks sputtering to a halt all up and down the road. On portages, on lakes, it doesn't matter to a Mack - they'll break down anywhere! (In those conditions).
You have to remember that up there you're above the tree line so there is no shelter from the wind, and the wind chill is a major factor. Many people say that the wind chill is only to do with the effect of the cold on skin, but believe me it makes a difference to mechanicals too.
-40c is sort of a significant point on the temperature scale, steel acts differently when it becomes that cold. It is much more brittle and much more likely to break. The City of Yellowknife, for instance, is very reluctant to grade the roads at -40c or below because the grader blades can simply start falling apart with only average use.
Back to the mechanical engines, the KW T800 that I drove for two seasons and the Pete I most recently drove both had the same 435E Cat engine in (one step newer than the 425 mechanical) and neither gave any cold related problems at all.
Ride Report: Canada North to South 2008 here
Drive Report: Ice Road Trucking 2005-2014 here