[joyridin']

Here are a few things I would check. This would relate mostly to "getting the car cheaper" than not buying it altogether.

Check inner and out tie rod ends on front
Check u-joints in steering column
Check rear brakes for wear...if possible, check rotors for thickness. They are a pain to change if you have to do a full brake job
Check front shocks.
Check ALL fluids
Check radiator on outside for deterioration

Be sure to drive it enough to get it to operating temperature and make sure you have no overheating problems. It might take a while for this to happen as they generally have a huge radiator. also, drive it around a few corner and make sure you do not feel any jumping in the rear end. This could cause problems.

A few other things if you have the time, but you REALLY need to know what you are looking for: Rear wheel bearing play. They have some play already, so you would be looking for excessive play. Pretty hard to determine if you have not felt a good set.
Clutch; Is it near the top? Again...kind of hard on these cars due to different master/slave cylinders used. E-brakes; Jag rear axles can be a mess if they aren't correct.

One last item: Check the body for stress cracks. Look around the area where the sidepipes come through the body, around the hood hinges and scoop, and around the fuel cap. They can be fixed, but you probably want to drive the car instead of having to invest $4k or more in a paint job and body work.

Good luck and whether you buy it or not, can you at least post some pictures?