These 100's are tanks, but even tanks need servicing. I look at under maintained as a challenge, it just parts and they are all replaceable.
Do your home work, starting with VIN. All Dealer work will be accessible, but may take a few calls.
Second, ask what a where work has been done, then verify.
Third, on inspection make sure engine has not been started for 12 hours. Then first look in radiator, then air box, and then start and listen closely note RPM through to normal operating temp. Bring something to lay on under the rig. Inspect for rust, leaks and damage. A very big clue to if it's been halfway maintained, is the propeller shaft yokes and spider joint. Are the dry, that's a bad sign. Or has grimy grease built up from being thrown off the lube points on to underside, if so it a good sign. Dry would mean the rig was not properly maintain. Dry propeller shaft & spiders joint (AKA drive shaft & U-joints) may need replacing. Possible a fully set of wheel & axle bearings & bushing as well, hopefully not the spindle. Assume anything not verified has never been done. If transmission, and gear boxes drive and sound good I not worry to much. If you do your own work it's just parts. If you have work done, buy one that has perfect records or has been baseline, it will be cheaper in the long run.
Forth, If not a experience buyer/inspector, at this point cut a deal based on third party inspection. Even the best will not find it all, extra eye are worth a few hundred.
The exception in my mind is if it's a no rust, NO NAV, no accident, good lines and cosmetically 7 or better out of 10. Just pay cash and drive off. ...LOL
By Zero NAV? I assume you mean NO NAV. Nice find.