Diesel, if your going to carry a camper, or heaver 5er or trailer go 1 ton. 3/4 tone will work just fine for everything else.
Airbags are something you might want to look into. I have a 12V and a 2012, last year for no DEF. Heard from others that use DEF and they say a tank gets you around 5k before you need to fill it again, Wal-Mart has it for less than $20 a gallon.
Now it's up to you on what brand...I say Dodge