I wouldn't start replacing all the sensors unless you're having trouble. There's a good argument for doing the CPS, since these do fail from time to time, and they're a nuisance to do in an emergency. You can then keep your old one as a spare. The O2 Sensor might be a good candidate too if you don't know its history, but otherwise, these sensors usually either work or they don't, and you'll know when they don't. The 91 and up uses a different fuel injection system from the earlier ones, and it's pretty reliable. It will send you a code when something goes wrong.
As for the seats, I doubt if those from a GC would fit without some redrilling of the floor, mount changing and so on. On the pre-95 Cherokee, the seat frames and buckets often crack, but these can be welded back together usually. When in good condition, I think there are few seats on any vehicle as comfortable as those in a pre-95 Cherokee, so I would make an effort to repair them or replace them with similar ones. If yours is a very plain model, you might do well to find a more deluxe 2-door, with cloth or leather seats and the higher-line seat base, which adds a tilt feature. The seat back adjustment is also a little finer on the deluxe seats.
Check your air cleaner for oil fouling, and if you appear to have a lot of blowby, consider replacing the vacuum line to the valve cover. They tend to clog up, and the system is pretty sensitive.
On an older 2-door, keep your eye on the top door hinges, for weld failures. If the hinge is currently in good shape, make sure that the door check mechanism also is in good shape, and get into the habit of not flinging the doors open fast, which stresses the hinges. Also keep a close eye on floor rust, especially above the exhaust system. It can sneak up on you.
120 thousand miles is a nice healthy middle age for an XJ. Enjoy it.
