Oh doG, I think this is going to be a long one.....
First educate yourself....all the way around....learn about the different models you are looking at, learn what they sell for, Tractor house is a good starting point, Then after you decide on a couple of machines learn as much as you can about them. Pick something popular, and start reading on them....learn what people say about them. The machines you have listed are newer machines, and likely still in good shape. If you want a 2320 that is very different from an 8n, and learning about them will be different. One is still looked upon as just a tractor, the other has been elevated to a collector status....that means there is a great deal written about them. All I am saying here is read up on what you are looking at....it could be real easy 8n, or a little more hard to find tons of info on 2320.
Then after you get a good idea and have looked at a few machines that are the same.....a few different 2320's you will get an idea on what they go for, if you start looking online don't freak if you see something way cheaper or way more expensive 1000 miles away.....I found out that like a house where it is can greatly effect its cost. Then once you have a few under your belt, if one seems too good chances are it is. Like with anything that little ole tractor with 4 hours fresh paint and new tires is like that little old lady with that super stock dodge the beach boys told us about. Good deals are out there, I passed on a 2yr old John Deere that had 1.4 hours on it....the old man passed away and it took that long for the estate to liquidate the tractor.....it does happen, but it was not a smoking deal.
Then if you are still leery about what you are buying, find that "guy"....if you have enough land to warrant a tractor you likely still have near by "that old guy".....you know the one that actually knows how to fix stuff, how stuff works, you will know him....he will be out every weekend on an old machine, but it will have new hoses on it and grease oozing out of fittings. See if that guy is game to help you go look for a tractor. See if he and his wife want to come down for dinner before, or perhaps he is just a six pack kind of guy. These guys are usually gruff, and want to just be left alone, but they usually are very keen on passing on what they know and are super nice guys...you just have to get past those defences.
Anyway that is my two bits....good luck to ya.