Great posts to this thread. My experience spans more than 20 Jaguar models, spanning some 50 years, and my current pairing of XK8 and S-Type is about to be joined by a late XJR. So, let me try to be bold enough to summarize the range of answers prompted by the OP's original question.

Like "how long is a piece of string?" . . . the answer is likely to start with "Well, that depends" . . . and it does! Unless you have been fortunate to have bought your X100 brand new some 10-15 years ago, then despite all the promises under the sun, its past is uncertain. This is why a fistful of itemized parts and labour receipts and properly stamped service book from a reputable Jaguar specialist place a premium on the value of these cars. On the other hand, being an exotic sportscar, many may have been flogged to within an inch of their lives!

Having said that, careful inspection is the first priority. In order, check for warning msgs and lights, scan for codes, systematically check all electrics, electronics and all connectors and ground or earth connections. Then, replace all fluids and filters (yes, including full auto kit). Next, check and repair/replace as necessary, all worn mechanicals - steering, brakes, suspension etc. Finally, focus on the bodywork and the interior. For the first time yesterday as I was dressing the seat leather I noticed a fine blemish in the leather (not yet a crease or scuff) on the squab of the driver's seat.

Why all this effort . . . and what is its reward? Well, that ole Mk1 in my avatar was my first Jag some 50 years ago. Both it and, I might add, all our Mk2s, the XK140, early S Type, the XJSs and even our magnificent Series 1 E-Type, did not have a patch on the build quality of the XK8/R. Only the gold Daimler VDP came close. All the former Jags leaked; they had rust traps hidden behind panels; they had far less to commend them than the modern Jaguars.

And yet . . . I recall precisely that stripping back that ole Mk1, we found the crankshaft to be within factory tolerance for new journals and less than half a thou oval on the pins. Rebuilding and restoring this beauty to showroom condition after it had already done 120,000 miles, we were rewarded with a further 280,000 over the next 5 years, during which our sole major expenditure beyond services and consumables (eg tyres, fanbelts, filters etc) was a broken windscreen!

So, go for it! The X100 is a fine car which, if bought wisely rather than as cheaply as possible, and then maintained carefully, will reward you with years of reliable and totally enjoyable driving.

Cheers
Ken