The newer the better...only because Jaguar had a reputation for discontinuing a model only after all of the previous bugs and kinks were corrected (later revision parts, metal timing chain tensioners, etc).

I see a handful of '06 examples with barely 100k miles going for $5,000 or thereabouts, majority of the 3.0 V6 variety.

My '02 (electrically an '01, built in May '01) is still in mint condition, and is driven only 3,000 miles per year (I work primarily from home), so I wouldn't mind a late model S or even a 2000-2003 XJ if mine bites the dust (new model anything is too costly to insure, the taxes too high, etc). And I can't justify a car payment for anything that will merely sit in a garage.