Can you get the serial number off it, photos, then post back here ?? Might be able to tell then or by looking if its a oil bath clutch, post '53 or so tractor,somewhere that was changed to standard from the dry clutch.
I would say it could be a few thousand, thats just based on ones I have seen, an exceptional tractor with a good undercarriage, oil bath clutch, direct start or starting engine, no final drive or major component problems, "could" and I say that lightly, fetch more. The 9U was a well built reliable tractors, with one of or the highest production run in that era, they made a lot of them.
What would be handy if you could source one is a Caterpillar track gauge, its a hand held template that you can place on all the various components and visibly see how much its worn, and compare against what was new. Other wise you will have to measure and then compare against new.
I won't venture to guess about the steering, I'm a little familiar with the brake bands on my earlier era D7's not sure, especially if this is a later 9U with oil bath clutch how that is arranged.
With track tension, I know later tractors, or sometime in the 50's they switched from the old style which required a big wrench, and often times cleaning and soaking those threads to get it to move to a grease type, not familiar with that, not sure if the 9U or later 9U has that.
I included the link for one site that had a description on how to measure an undercarriage, the photos are gone now, do some research on the net, youtube or similar, you may find something, lot easier to post a link to something like I did then to try and explain it, but know that eyeballing the components is a mistake, to determine actual wear. Rail height, and if the pin bosses are hitting the bottom roller flanges, sprockets worn to a sharp point, can be seen, so you can suspect what is worn sometimes, but it should be measured to be sure if nothing like that stands out.
YOu can dip a magnet in the final drive compartments, see if there are metal flakes or bits of metal, if this has bellows seals, finals can leak oil and what can get out, can get in, they can leak from age or lack of use. Not sure if those changed from older tractors, they were common for years, also you can pry against the sprocket to see if there is any play, if so, that can be a problem or indicate something may be wrong, you don't want to run them like that. These were referred to as bullet proof tractors, so if its sound, could be a nice find.
Starting engine is nice for cold weather starts, make sure to get that, if were mine, because of where I live, it would go back on. These like clean fuel, hot spark and clean oil, some were known to have the carburetor leak down fuel and contaminate the crankcase oil, not sure with the horizontal opposed piston type engine on this, but do be aware, pull the dipstick, check for thinned or contaminated oil, change immediately if so. These when in proper tune and within wear tolerance, (good compression) will fire and run nicely, and you have almost unlimited time to start that diesel. The advantage is, the coolant warms up, (some of these have to have the diesel engaged to circulate coolant to the starting engine, if not you will overheat it, I forget if that one is like that)oil is warmed and circulated, oil pressure is built up, all before it fires, essentially conditioning the engine to fire in cold temps and have oil pressure before it fires, I am pro starting engine, my D7 will start with the handcrank if everything is maintained and in tune, magneto needs to be firing a good blue spark, and timing set right, spark adavance. Most of the reason why people get fed up with them is because they do not take care of them, these were designed for a reason, and I think should stay with that era tractor, more so if operating in cold temperatures, I need no ether to start my D7 in bitter cold, it will and has fired up at close to 0 deg F, direct start would be a pain with an engine that may have some wear. Just some thoughts, there are some folks here who forgot more than I'll know about these, that participate here. I'm sidelined today, skinned my knee but good yesterday, on the injured reserve list today.
PS - undercarriage parts availability, I am not sure if Berco, or even CAT offers any parts, depending on what pitch, and what conversion might be available, OEM or aftermarket, it was high production tractor, and later series may be compatible, older D6's were offered in 2 gauges, 60" narrow, and 74" wide, (measure from center of track to center of track) and now I am realizing the 8U would be the narrow gauge and the sister to that 9U, so that gauge changed after the 9U and 8U series. Other parts may be available through the Cat classic parts line, salvage and similar. The manuals you need must correlate to the serial number runs and are title as follows, Operators Instructions, Serviceman's Reference Book and Parts Catalog, best ones are good used OEM and I will bet you will see a bunch listed on ebay, its where I have found books for everything I own, most of which is old junk, LOL well not really, but you get the point, older but functional.
Good Luck with it if you get it !!!!
Hope this helps!!!!