Todd:
I have some experience with old XJ8s, having owned two XJRs which I bought each with about 150,000 miles and driven them to 230,000 + miles or so. I also maintain my wife’s 2002 XJ8 which we bought new and now has 160,000 miles or so.
First, let’s analyze all you have actually told us about the situation. You have identified an XJ8 which the owner cannot or will not give even the most basic service history on and… It must be pretty good looking because you are still interested. You did not mention price or what is in good condition and what is not.
In my anything but humble opinion, how an old Jaguar looks is by far the most important issue IF YOU ARE GOING TO DO SERVICE YOURSELF. Paint, body corrosion, upholstery, and interior parts are prohibitively expensive to fix once they reach a certain age and look. The engine and transmission are certainly up 250,000 mile or more service life, but sensors, throttle body, fluids, belts, hoses, tensioners, fuel pumps, and engine auxilliaries are all reaching the potential end of their life at 150,000 miles and this much age, BUT (in spite of what some around here would have you believe) the XJ8 Jaguar is not significantly more complicated or mechanically sophisticated than other high end automobiles. In fact, as I used to point out, my daughter’s Chevy Prizm had variable valve timing, EFI, Electronic transmisison control and so on. You get the point, I assume.
Having the dealer, or even an Indie, work on a Jag IS, however, generally more expensive than the cost for another car because the parts are less readily available, the information is less available, the diagnostic equipment is less abundant and so on. And they can charge more.
So, if you are going to do much of the middle level work yourself, and are able to do without the car as your primary mode of transportation while you figure out the problems, wait for parts delivery, or have time to work on the beast, the XJ8 can provide a wonderful car at a reasonable cost.
If not, assuming you are in the US, I recommend you buy a Taurus or a Crown Vic.
Just my $.02! Other (wrong) opinions might vary.