I've got a 2015 2500 with the CTD. Great engine. Its quiet and very efficient. Empty I can get 20 on the highway. Towing 9500 lbs trailer I can get 11-13 depending upon speed and wind. I usually tow at 65-68 mph.
DEF is super easy and relatively inexpensive. DEF is DEF is DEF. Brand name at $10 a gallon or truck stop at $2 a gallon is the same exact stuff. I drive about 10,000 miles a year with my truck, most of it towing. I go through about 10 gallons of DEF a year. Its a non-issue for me. Whenever my gauge shows half or below, I'll grab a 2.5 gallon jug and fill it up. You start getting a warning when you have about 200 miles from empty as well. Truck stops, Walmarts, decent gas stations, auto supply stores, Amazon, etc have DEF. Its a very simple system, at least as far as your input is concerned.
No problems with the engine or base 68RFE transmission either. Plenty of power. Exhaust brake is awesome. Put tow/haul on and exhaust brake to on and it will really slow you down in a hurry. No T/H and exhaust brake on auto isn't nearly as aggressive.
I love my Cummins. The Ram truck around it isn't bad either.