Hello, another new member here. I've done quite a few installations so I'll try to answer your questions :

1) I don't know if an XJ sat/nav system differs from the X-Type, that's out of my knowledge, but they look alike at first sight.
2) You will need the screen and the casette/CD/MD player above it, a remote climate control module (RCCM) and a DVD drive for the system to work. If you like you can add the CD changer, BT and phone module etc.
3) You must have a custom sat/nav loom and optical fibre cable. If you can salvage the whole loom from the donor car, I can help you modify the loom or if you'll get only the connectors, I can help you build your own loom. If none of these are possible, I'm building and selling those looms but since I'm a newbie, I'm not allowed to post an ad here. About the optical cable; if you don't have anything related to sat/nav and multimedia in your trunk, then you only have a female optical connector on the left side, behind the wall carpet. You must buy a cable according to the number of devices you'll hook up. If you'll attach a CD changer as well, your optical cable must have 3 connectors daisy chained. 1 for the DVD, 1 for the CDC and the last one for the car's original connector. Here's an example of an optical fibre cable for three devices : Jaguar X-Type Link lead - CD Player and SatNav (C2S50385) - JustJagsUK.com