I had an 05 Super Duty 6.0 PSD with ~170k miles when I traded it on my current 12 Super Duty 6.7 PSD. My current truck is a little over two years old and 50k miles and it hasn't experienced a single hiccup, but so did my 05 until about 80k miles when it required an EGR valve cleaning.
Comparing the two different (unmodified)trucks the 6.7 PSD pulls much better and makes better fuel economy, and just run's like a scalded ape.
It sounds like your looking at used trucks and the single most important feature to make sure it has no matter what brand is DEF/SCR equipped. This feature alone promotes better fuel economy, better overall power, and much less fuel/oil dilution of the engine to name a few.
Good luck...