Welcome to the forum. I haven't owned an XJS, but I did own a '93 XJ6 which also was a great car. Which engine do you have, first of all? The 6 cylinder engine gave me only two problems. The air temperature sensor and the oxygen sensor.. when these go, you will think your fuel system is dying a slow and painful death. Luckily the parts are inexpensive and easy to change.... I am not sure about the V12, hopefully the same could be said. I recommend changing them out unless you know that they are fairly new. Also if you have inboard rear brakes (I can't remember if your car does) check the pads right away. If these wear and cause rotor or caliper damage, the full inboard brake job is costly and difficult for most people. Beyond that, I can't think of much, maybe treat your leather with hide food and keep your car garaged if at all possible... Jaguars and moisture don't generally make a good combination, hence all the electrical system nightmare stories you may hear. Enjoy your XJS, beautiful machines.
Brian