Hopefully a certain party that knows these quite well sees your post and comments, both what to look for on these, as well as parts support, the latter of which you may want to thoroughly check out.
Undercarriage should be measured, though some visual wear can tip you off, pins/bushings, roller flanges hitting pin bosses. If the seller says its got 70% left, meaning the track components have 30% wear on them, this should be determined by correctly measuring each component. eyeballing these parts resulting in a verbally communicated assessment to a buyer, talk is cheap, show me the measurements or I'll need to do it myself. These parts, though a 450 is a small grading tractor, can be expensive, a lot more on larger tractors, so if you pay full used market value on one that you were told was in a certain condition, that measures out to a whole lot less, you will not be so happy with what you spent your hard earned money on, would be better off renting in many situations.
There are some specifics to know about these, mechanically. I've run 1010's and 350's and 450's, at a few places I worked, someone always had one or two of these around, from an operators view, they were fine, maybe not so much power, but you work with what you have, work still gets done and for what we used them, maybe it took a little longer, seemed reliable too, but you will want some more detail on specific parts of these to look at prior to buying one. My neighbor has a 350 or 450, I can't recall now, its been sitting a few years, something with the reverser if I can remember, and I seem to think there is a problem, cost, availability or something, he'd be using it if that was not the case.