I owned an '88 XJS and it was my daily driver for several years. It was a maintenance nightmare, and mine was relatively low mileage. There's lots about this on the web, but I had to clean up the distributor to free up the weights so they worked properly. I did the Lucas/Delco alternator switch and a number of other things to make it more reliable. Had to drop the rear end to do the rear brakes, and replacing some cam cover gaskets was a three day job to stop a 2 quart per week leak. Changing out all 27 or so cooling/heater hoses was another nightmare. Changing spark plugs was a half day. The byzantine vacuum/emissions stuff was poorly designed and I could never get out some little bit of throttle hesitation when cruising. Then there was the engine fire. $1200 per year is a light number, in my experience, unless you do all your own work (as I did) and only drive it occasionally. Still, was a magic carpet ride car when everything is working and got me lots of attention. Some of it welcome. :)
General rule is that the younger the car the better. Prior to a change in the head in the early 80's, the mileage was less than abysmal. Once Ford bought Jaguar in the early 90's the quality improved, but it still wasn't great. I'd run away from a '78, especially at that price. There are younger ones in similar good condition for less. I eventually donated mine for a tax write-off as I couldn't sell it. And it was running and driving well at the time with no cosmetic issues and all the above items fixed. If I were to buy another one (and I won't) I'd buy one of the few 6 cylinder manual cars that are out there. I think there is one for sale in South Carolina at the moment. Or maybe a V8 converted car, but what would be the point of that?
I looked at Jensen Interceptors before I bought the XJS, but found choices were very few. I looked at several and they were all poorly maintained and required a lot more work than I was willing to do. They aren't worth restoring, so the ones that are available are pretty worn. The only ones with collector value are the convertibles. I saw several really nice ones at a big show in Canada last year, but I think they have gotten pretty pricey. The XJS was much more plentiful and had great parts and enthusiast support.