Yeah thats really broad but in general, in my experience, older diesels are where its at. The electronic engines are great when they run, and harder to diagnose and repair when they dont. Emissions engines (after 05) you can expect to work on the emissions system alot.
I have a 96 12v cummins....which is all mechanical, no electronics. My dream engine, parts are readily available as there are thousands still on the road, and thousands more being rebuilt and used in aftermarket projects.
If i was looking, a 12 valve 5.9 cummins, a International DT466 or DT466e would be my engines of choice. The DT466 is probably the most prolific mid size diesel engine out there. Those are located in 2005 and older. You would have zero problems at all getting parts for these engines for the forseeable future.
Not doing the work yourself? Got a mechanic you love and trust? Ask him what he is comfortable working on and familiar with!