To add to what Exotexs said, the X350 is I believe the best of the new (Ford) Jaguars. I am also certain that the lighter aluminum body was introduced in 2004 or so, and is still receiving high praise today. The most recent redesign is mild, the 2008 that is, but all of the XJ line is still using the aluminum body design.
I tend to agree that a later Series III is a great Jaguar. I have had three in total, but one was a poor example to say the least (not any fault of Jaguar, just not maintained by the original owner). However, if you are looking for a modern Jaguar, I think it best to stay with the X350 range, the reports on the 1996 thru ? X300's were varied, a beautiful car, but with its unique set of issues as Exotexs mentioned. A well-maintained X300 should be a fine car, read the reports on troubles others have experienced and prevent these from happening to you by doing the repairs before the repairs do you... as with any car I suppose.
The X350 is a more solid Jaguar and I believe that the company took all that was great about the X300 and improved upon it (styling, features, the classic lines) and at the same time listened to the owners and redesigned the components that had been troublesome, giving Jaguar its best XJ model since 1987. What I admire about the X350 is the classic Jaguar design, the interior that is a modern progression of the XJ's styling, and the reported reliability of the cars. Do a carfax, check ebay for an idea of the going rates, this should also give you an idea for repairs that need to be completed based on the ebay sellers list of recent work and the mileage of the car. Motorbase didn't give me alot of detailed information on changes in the XJ line, but try
https://www.jag-lovers.org/x350.html , this is a great source of information on the various models of all Jaguars. And apparently, 2004 was the first year for Aluminum bodies, according to the information here. Hope this helps!
Best Regards,
Brian