Get a subscription to Diesel Power Magazine or any of the others and learn what to look for, what to expect, what to avoid and how to preempt potential failure. Draw your own conclusions regarding what you want.
As mentioned above, one of the things I would personally avoid (if I knew) is buying a diesel truck that has had a tuner put on it at some point in its life. Many late model used trucks that have had the DPF removed would need tuners to bypass this emission control system.
Then there is the current recall on FCA vehicles that you would need to consider.