Modern engines last a very long time with little care (gas or diesel). It's nothing to get a reliable 200-250k miles out of an engine with no significant work.
We replaced a 1992 F250 2-3 yrs back. The truck ran fine and I wouldn't have hesitated to tow cross country but the body was falling apart due to midwestk rust. It had 265k miles when we sold it.
We replaced it with a 2001 F250 with 110k miles and have put on 40-50k miles since then.
I think the sweat spot is 7-10yrs old. Lots of miles left for 1/10th to 1/4 the purchase price of new and you should easily get 6-10yrs out of it.
