First of all, the Kelly blue book values tend to be low. Check the NADA values for something closer to what the car is really worth.
On any XJS the first thing to check for is any sign of running warm. The temp needle should stay on , or below the N. Always. The next thing is does the car have a fat pile of maintainence records. The XJSs are tough, durable cars, but with a V12, they have two times the maintainence requirements. They will not put up with overheating at all, and the consequences (dropped valve seats) are really expensive. Look to see if it has a genuine Marelli distributor cap.They last about 30k miles, and the cheap repros don't last as long, and are prone to failing, leaving the car running on 6 cylinders, and melting the catylists on the dead side. Look at the inside edges of the front tires for wear. Check for rust on the rear quarters, and the bottom of the left front fender. Check the seats and carpets for wear. The AC should be ice cold, and transition smoothly to heat.The car should run very smoothly, and waft you down the road. They are not quick, but they are fast and smooooth. The last thing that I can think of right now, is look for evidence that the rear pads have been changed. They are inboard and if forgotten, the rotors are a major job to renew. Hope this gets you started. I own two XJSs and think that they are fine cars, but like all mistresses, they make some demands on your time and money.