I own a 2003 Duramax that I am very happy with. If I were to buy a diesel now, I'd look for a pre-DEF without question. You may be able to find the last of a new 2012 Dodge still, if you're lucky.
In the used market, my knowledge is limited, but if it were me I would stay away from Ford 6.0 engines, even though my understanding is they can be "bulletproofed" with some aftermarket work.