On track loaders you want to stay away from any case machines and also any 655, 755 and 855 John deere's made in the 80s or 90s. A 555g John Deere would be about the only deere track loader that would interest you, and still won't suit you like a rear engine cat. About any 943 or 953 is fine, they both have them same engine, a 43 is 80hp and a Pre 1992 53 is 110hp, 43 weights about 25k and 53 weighs 32k, both operate exactly the same.
Hours on older equipment are often not right, so you are mainly looking at how it operates and the overall condition. Things like, alot of blow by, air bubbles in the radiator, water in the oil. You want to make sure it runs in a strait line and will spin the tracks when you push against something solid.
There's alot of things to look at, have someone look at it that knows equipment and that's all you can do. You could buy a older machine capable of doing your work for 10 to 12k, but if it breaks your putting money into something that won't make you anymore when you sell it, if you have to repair a little nicer machine you will be increasing the value for resale.