I have rebuilt a couple non-turbo IDI and one turbocharged. Really a bulletproof engine, often the pre-combustion chambers that protect the glow plugs fail, not difficult to replace but heads need to come off. Also injection pumps wear and need to be rebuilt/exchanged, but they seem to run forever, even when they get ugly a shot of starting fluid (not recommended) but I have seen it keep them going for years. I have seen 2 with rusted out oil pans as well
Trannies can be weak, seems like a junk one stays junk, and good ones last, not an AT tranny guy, but I heard cases wear, but if it shifts good, it's likely good
After that, ball joints and radius arm bushings often fail, leaf springs can break if mistreated early in life, but pretty tough trucks overall