How new of a used diesel are you looking? None are perfect, but there are those to steer clear of.
The Ford 6.0 and 6.4 are to be looked over VERY CAREFULLY. unless you can turn your own wrench. If not best to steer clear.
Duramax had some minor issues in the early years, but the 07 LBZ is the holy grail of the Dmaxes.
Ram the Cummins 5.9 is one of the most reliable diesels in a HD pickup. the early 12 valve had a issue with the Killer dowel pin, I would place a bet most have been fixed.
The Gen 2 24 valves had issues with the Bosch VP44 injection pump, corrected with a larger after market lift pump.
3rd gen had some early injector issues.
My signature has what I drive , and would buy another in a heart beat, they seem to be going up in price lately.
Good hunting.