Yeah, for $20K it better be the very best XJR/6 in the whole world.
I drove an XJR/6 as a daily driver for many years. Fantastic car in many respects; I really enjoyed it. Jaguar got a lot of things right. Engine and trans are very durable. I sold it at 171k miles (I bought it with 30k) and the engine an trans were still solid.
But, to be honest, it wasn’t always a pleasure cruise. Differential failure at 77k miles, wheel bearing failures, coil failures, various driveability and engine management issues, idler pulleys failing, ABS faults, multiple radio problems, seat heater failures, and a slew of other issues. To be fair, many of these things were when the car was well over 100k miles so it’s expected that more items will eventually give trouble. And, never once was I left stranded by the side of the road so, in that sense, it was very reliable.
The X300 cars enjoy a good reputation in general but age and miles will eventually take the gloss off any car. Since you’re considering a very low mileage car you might be spared some of the problems I had.
Some of the engine management electronics require specialized equipment for resetting/recalibrating the system. When I owned my XJR there was very little aftermarket/indy shop equipment available to do so, leaving no choice but to rely on expensive (and in my case, distant) dealer visits. By now I reckon many dealers no longer support the old X300 cars…but perhaps the aftermarket has caught up?