They have totally crap electrical systems and wiring, so damn bass akwards electrically complicated that they usually end up siting in a garage space collecting dust once they develop phantom electrical issues.
New off the showroom floor in 1990, you were lucky to get 50K miles before they were stuck in the shop.
When they say it was garaged and sparely driven, there's a reason! You are about to be the next sucker.
That unit probably has a parasitic battery drain issue, one that they can't find so they are dumping the car.
Connect a test light between the battery + and starter cable. If it lights up, you don't need those problems.
The 1990 XJ6 is prone to wiring connection corrosion, exacerbated if you live in a salt water environment.
People will often convert them to a GM 305 / 350, they do sell full kits to convert them to reliable GM parts.
XJ6 with a 300HP Camaro LT1 / 4L60E drivetrain - http://www.jaguarspecialties.com/photos/XJ40w-AC-2.JPG
My advice is for you to stay away if you're not good at finding scrap parts and installing them yourself.
A water pump for that car is $300 and hard to find, a GM SBC water pump is everywhere for only $50.
If you break down, it's flatbed time and you better have a ford fiesta to get you to work for a few months.
The same goes for Rover, they are outrageous to get repaired at a reasonable price. Highway robbery$
If you plan for a street semaphore sleeper car, it could be a very fun project car to modify with patience.