Further to Brian's excellent reply (who put me straight when I bought my first XJ8 ignoring anything earlier than 2001 build just to be sure of not getting a Nikasil engine) I had my timing chain tensioners checked by Hyper XJ (now retired) in Thame. He reported that my Sport appeared to have been owned by an enthusiast as they'd already been changed. Cost just £48 for the check and a note confirming. Nothing in the docs when I bought
Had previously my waterpump changed (impeller hadn't shattered, just was slipping at about 108k miles so temp was going up and down) by Ray at Surry Jag Centre -- not the cheapest but effective -- latest new metal type pump installed. And coolant checked for gases after flush out and refill. They also did a rear suspension check and found gravel (having been assured it was "rear wheel bearings, sir!" by a Jag main agent -- removed

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Had the Exec's (aka Ludlow, aka the eBay bargain) top timing chain tensioners changed (no luck there with them being done before) at £450. Both those tensioners had vertical splits in the green plastic so were on the way out. Price at Hyper XJ was with an ATF and fluid change, and a check of brake fluid for air . You really should have those two done as Brian says. On my Sport the ATF fluid had been changed before (it was dark orange). But on the Exec it was filthy black and was the original according to mechanic.
There are some decent indies still out there, but they need tracking down from previous recommendations.
Looking back in the early service history for the Exec I found that main agents had changed a Cat (at less than 30k) coz the warning light for emissions had come on. A few thousand miles later they changed the sensor. Total cost to the then owner? Over £800 folding. Despite there being a method available for actually checking sensors' correct operation
Good luck with your XKR -- although low mileage get the check done to ensure it hadn't been run on sulpher fuel the majority of its low miles. Before my first XJ I spent about a year trawling thru adverts for an XJR -- they were either very good (one I nearly stumped up for had been one owner Royal household and serviced in Kensington Mews but they refused to accept the agreed price gazumped at the last minute) but too pricey. Or were average priced but obviously had been ragged as an XJR should