As someone who just finished an exhaustive search and bought a DP within the last month, here is my perception:
1. Pre emission control (pre DPF) probably pre 2007, are reliable engines. Most of the available engines, both cat and cummins, had long track records.
2. About the time emission controls became required, there were some reported problems with the systems. I received some advice to stay away from year 2007-2009 for that reason, but it wouldn't have stopped me had I found a coach I liked.
3. The most modern engines, say 2013 and newer, do run cleaner and get better fuel economy. Sound like most of the bugs have been worked out.
I ended up with a 2005 simply because it was very clean, well cared for, and had the floor plan we liked. (and we could afford it).
2005 Discovery 39s Cat C7 330hp, AF1 brakes,
2013 Silverado Crew Cab Toad, Blue Ox Towbar