Good to hear such longevity in that model. Looking to purchase a used XJ8R or 8L, both have around 140k miles. I probably won’t put on more than about 6000 miles a year, just want something comfortable and nice looking - and the bad reputation seems to be overstated so I’m not too worried.
I strongly advise staying with the 6. It is the classic, bulletproof XK120 engine. With all the other things to worry about you will never worry about the engine. There are lots of Xj6es around from the mid 90s that are gorgeous and amazingly affordable. (They terrify people...)