Nick
I believe that the decision you have to make is entirely a personal preference / decision. Taking your time to decide is always a double edged sword. You can miss the best you have seen in the interest of finding something better. I personally would not buy a high mileage Jaguar, the likes of what you are quoting of around or approaching 100K miles. I also prefer petrol engines as although they might be a bit heavier on fuel, I do not believe that in the long run a diesel is any cheaper to run in the long run when you take into account servicing costs etc, etc. The fact of cam chain instead of a belt also gives me more comfort.
It does though boil down to "each to their own and personal choice." There are members here that will disagree on my point of view / preference and will thoroughly recommend a diesel engine. As I said, "each to their own"
Best of luck with finding the right car for you.
Regards, John