Not much I can add to what the others said, but that is a really nice example and it sounds like it would be in the best shape you could find in one of these, having been 1 or 2 owners. With that in mind, I would think this S-O-S transmission in it would be in good mechanical operating condition, aside from any maintenance work or adjustments, latter if any problems are noted, the manuals and folks here certainly can help with that. I would suggest getting the correct manual for this tractor, very good thing to have.
I had one of these for about 10 years, a 1964 model that was a former NYS thruway mowing tractor, it saw a ton of hours then and a fair amount when I had it. We did about everything with it on the farm, cut rake bale hay, snow removal, some tillage, lots of rotary mower cutting, mid mount sickle mowing and whatever else. It operated flawlessly and sure you may notice the hop when upshifting and or letting up on the inching pedal, this one was mild in that regard. We sold thousand series new with S-O-S transmissions and our mechanics were well versed in working on these. I did not see many in the shop, as was said, they were finally proven and even with those who were not so keen on maintenance, they held up very well from what I saw, and experienced with the one I ran.
With maintenance and care, given the speculation that it should be in decent condition with lots of hours left and then some, you'll more than likely get all the use and a lot more than you'll need.
If you add a loader, consider like may have been said already ( did not have time to read all posts thoroughly) the front axle, specifically the pivot pin assembly on this adjustable axle, and maybe even the spindles with heavy work. As much as this front end is not the best suited type for the application, it will hold up, but over time the wear to the pivot pin will be accelerated and that will be a maintenance item, longer you let it go, the worse it gets and they will run until they are very loose with excess free play. Also, given the way the S-0-S operates consider all the back and forth between R1 and or forward gears, when doing loader work. You have what appears to be an outstanding example of one of these, in my opinion, you may accelerate wear to it with a loader vs. leaving as is in which I would be very confident with the minimal maintenance, as is, should last you a very long time.
I was very fond of the '64 4000 because it had power steering, Independent PTO, 1 set of remote hydraulics etc., just an easy tractor to operate and was very stable on the hills, an LCG version would be even more so. I ran an 850 with a loader and a 6' rotary for 14 years, and while I did use the loader quite a bit, the only handy thing about it when mowing was being able to push a fallen tree out of the way. You may be $$$ ahead with a dedicated loader tractor, but entirely your call just the same, as having all in one is also beneficial, no doubt about that just the same.
Fluid, no matter what you do and how far you have go to get it, one thing is certain to keep it operating, use the correct modern equivalent and verify that it meets the correct specification for the S-O-S transmission, or get what CNH has that meets same.
If you put the wrong fluid in this transmission, it will fail soon after and this will be a time consuming costly repair. There is no taking chances with this, just wanted to make that clear.
Great looking tractor you have there, wish you the best of luck with it, I enjoyed the heck out of the '64 and would not hesitate to own one in the condition like yours !