If it doesn't leak oil profusely, or have water in the oil, there is nothing else to know. It'll roll for a long time if you tend to it properly. I have an XJ with a 4.0 Auto with 240k. Still holds 40 psi oil pressure. Use NAPA gold oil filter, WIX, or Ford equivalent. Fram sucks, Purolator sucks, generic sucks.
89 will have a Carter Carb and you will have a few different options for trans and Tcase in that year. Still not a bad deal for the right price. 91 and newer will have the fuel injection. The 89 will be more like a CJ7 in the body of a YJ. It will have the sport roll bar and a few other things diferent from newer. Mine is a 90 which is similar. If you don't mind the carb or spending a grand down the road on a fuel injection kit than it is a good ride.