From the start, the XJS came with a V12 engine. It originally came with both manual and automatic transmission, but the manual was soon dropped. This surely disappointed some Jaguar purists, but it was most likely a decision made by market demand, and the luxury market tends to prefer automatic transmission.
There were three generations of these cars. They were called the Mark I, the Mark II, and the Mark III. They were produced from 1976 through 1980, 1981 through 1990, and 1991 through 1996, respectively.
The Mark I could reach speeds of 157 mph with a 0-60 time of 6.5 seconds, making it a competent sports car. The Mark II one-upped those numbers, and it did it while using a more economical High-Efficiency engine. The Mark II could reach speeds as high as 168 mph with a 0 to 60 of 5.7 seconds. Those numbers made this car the fastest automatic transmission production car in the world.
These cars are slowly becoming collectibles, so there is now time better than now to buy one. The least popular models are selling for just a few thousand, with the older convertibles fetching $20,000 or more. I suggest seeking out a mid-80's models. They had peak performance and tremendous quality. They have about as much bang for the buck as you'll find in the current used car market. Try finding one on eBay for a good price.