I would consider new.
DEF is NOT an issue. Every 3-5k miles pour in 2 1/2 gal. Takes about 3 minutes.
As mentioned the new ones have new heavier frames and higher payloads. 14k for DRW.
Your looking at a Laramie, but if all the luxury items are not needed, you can get a brand new 16 Tradesman 4x4 CC DRW with Aisin trans and 4:10 gears for $46k. And you get full factory warranty and not have to worry about an unknown history.
With the price discounts at the volume dealers, it almost doesn't make sense to buy a used truck.