Case are the easiest to find used parts for as there seems to be more of them parted out. Ford is the hardest to find parts for as there are not near as many ever made. JD and Cat will be the higher priced ones.
You did not mention your price range. That makes a world of difference. If your looking for some thing in the $5000 range you are going to have to get a real old Ford or Case to find anything that cheap. If you can spend $15,000 then you have a lot more choices.
Also it makes a difference how often you are going to use it. I started out with a three point mounted Kelly. I did not think we would use one much but I soon found out we used it quite a bit after we had one. So I sold the three point one and found an older JD 410. I have had it 15 years or so. It is has MFWD and a cab. It had 2500 hours on it when I bought it. At that time there was a bunch of them around used I was able to buy it for $17,500. Just a few years after that the building trades took off and it would have cost $30,000.
So the general economy makes the value of them go up and down as building demand cycles.
I remember reading thirty years ago how they where hard to find because the used ones where getting shipped over the Kuwait after the first Gulf War.