What Tow Vehicle is Enough?

If you pulled it with an 02 6.0 and you were good with the performance, given what is available currently that is certainly the low water mark. A 2011 or newer GM 2500HD or 3500HD are excellent. Improved brakes, more power than you had before, 6 speed tranny and more HD frame and there comfortable to spend all day in. A Ford would give you much the same. As for dually, yea if its a dedicated tow vehicle why not. You don't "need" it at that weight perhaps but if all its going to do is tow you might as well spec it for that purpose. I pull the same weight with dually and it is as good of an experience as you can get towing 15K. Last as to diesel, you obviously don't need it, but much like the dually option, its a better experience at that weight. The 6.0 GM and 6.2 Ford will move that weight in an acceptable manner. I pull that much and more with my 6.0 at likely 3X your elevation and it pulls well. As to make of diesel, IMHO avoid the Ford 6.0/6.4, other than that your chances are good of being happy with whatever make you select.