It all depends on the price, that's for sure.
It also depends on your willingness to learn and ability to work on one of these(do you have other transportation, or will it be your only rig?)
I'm pretty well convinced that most of the obvious issues aren't deal breakers... you just need to be aware of it going in.
Hard starting IDIs are, well, most of them. Glow plug, wire connector and/or controller replacement and you'll be good to go.
Smoking badly at idle, or when cold, either gray or white? Timing/injection pump generally.
If it's also got a dead miss/skip that doesn't go away at higher RPMs or when warmed up, then you worry more - Probably a rocker loose or valve screwed up.
Valve cover oil leaks? Typical. Usually just need to tighten the bolts down. Or replace the gasket for just a few $.
Overfilled oil? As long as you know that the person deliberately overfilled it(say by checking it with the engine running), OK. A couple of extra quarts won't make much difference. Otherwise, it could be leaking something else into the oil, which might require a headgasket to be replaced.
Oil in the coolant? Oil cooler needs rebuilt.
Coolant in the oil? Head gasket or valley pan gasket.
Leaks oil? Well, yeah... They all do that.
It's International's patented rust protection system!
