It sounds like it may have a well worn undercarriage, but you really cannot determine that unless you can measure the components and see what the wear measures on each.
I don't have the specifications for "new" on an RD series tractor, someone may be able to help on ACMOC, or ACME, antique caterpillar forums.
One thing that you could check is the track adjuster, its the old manual type, and see if its all the way out, I'd have to look in the closest book I have for that era tractor, you can measure and see where its at, max or still some adjustment left. If the pins and bushings are worn significantly, this is the part of the track chain that rides in the root of the sprocket, and has an unworn side, they can be turned, and the the new unworn side used, used to be more common to do this in the past, you would have to split the track, remove and take to a track shop to do that.
The simple way to eyeball the undercarriage is to take one of the old track gauges, Caterpillar used to offer them, not sure where to find one of these, sourced one for my old D7 on ebay. You can literally place the gauge on each component and see what the component should be new, then deduct what it shows. The track pad is an easy one to use for example, just place that part of the gauge on it, and observe how much is worn off the cleat (grouser) on the pad, if its worn 1/2" there will be a space of 1/2".
RD's are not especially common or extremely rare, nice size tractor, and I believe that one to have the 3 cylinder diesel, which has a unique sound.
If this tractor was just used for drawbar work, and or agricultural use, likely its not all that abused or worn, if it has a dozer kit, it may well have seen a lot of use and more wear. After re-reading, you said it has a blade, so you'll want to look it over, likely it has a LeTourneau power control unit, PCU (cable) and dozer kit or similar of that era, possible to have hydraulics, but if not, you may want to look that PCU over, some of the friction parts like the driven and driving cones are hard to find. If it has a Cat CCU cable control unit, which were not built by cat until many years later than the RD series, you may be able to find parts if needed.
You can pry from the sprocket to the final drive case, to see if you have any final drive issues, starting with a loose sprocket, can dip a magnet into the final drive oil compartment to see if there are any metal fines or flakes. I mention this as besides potentially worn tracks, final drives require specialized tools and a heavy puller to disassemble, can be expensive to repair.
Generally speaking you want to look the tractor over thoroughly, measure track components for wear, notice any serious problems, suspect welds or what have you.
These were well made tractors, and quite reliable, its possible you have found one with a well worn set of tracks, but still a good runner with out other problems or any variation thereof, a good runner with worn tracks, can still be very useful on a limited basis. It can get expensive and challenging to find parts, but not impossible. I'd check it out carefully, it may be worth scrap price if well worn.
One thing about the starting engine, is to make absolutely sure the oil is clean and no gasoline has thinned it at any time, the zenith carbs on these can leak down and literally fill the crankcase, not always or not everyone, but I have seen it happen, run that engine like that, it will toss a rod or destroy itself in a hurry, CLEAN OIL is important, its splash lube, with a deep sump. You really need to look after these to make sure they run right, clean fuel, lots of it, sediment bowls get packed with rust from old fuel tanks easily, so does the in line screen on he carb. Another thing is coolant, if you run one of these without the diesel spinning, no coolant circulates, it will overheat depending on how long you run it, the air temperature, not as fast on a cold day, you have a little time if fooling with it, but not a long time. On later D6's the engine was a 6 cylinder with the other type starting engine, horizontal not sure if the same rules apply for coolant, but do know the vertical starting engines you have to spin the diesel. Some things to know about these, they like clean oil, plenty of fuel and spark, don't overheat them, and when the diesel starts to fire, make sure it does not overspeed the starting engine, the pinion has adjustments for the latches to release centrifugally. You can easily hear it if it does not release on time, throttle it back by hand, choke it out or similar. Again, just some basics to keep you out of trouble, if you are not familiar with these starting engines, they work great when kept up and will get a tractor started in bitter cold conditions.
I'd look it over and see whats what, carefully, before I'd offer anything above scrap value, too much wear, may not be worth it, but if its complete, tracks not falling off, runs well, no other major problems, leaks, loose sprocket, or final problems, it has a dry clutch and the pilot bearing needs to be kept lubed, not sure if that one has the inspection cover over the clutch, you may be able to inspect it, probably the weakest part of this era tractor, just a wear component, that requires periodic adjustment, late tractors came with oil bath clutches. I may have been redundant with some of this, hopefully its a tractor worth owning, I'm a big fan of that era Caterpillar, even well worn ones with some life are very useful around a farm, just don't turn sharply on a side hill in deep mud, thats usually where a track wants to come off, and a RD6 is large enough to call for some hard work to get the track back on, we did it once on the older of the 2 D7's, and it was coming off a slope into a wet area, took some doing to get it back on, likely your's may have similar wear, that tractor was still very useful, especially when a wheeled tractor got stuck.
