Replacing the stereo is straight forward, just order the correct fascia kit, wiring loom and tools to remove the old one. I'm leaving the 6 CD Sony unit in mine as it has an aux input in the glovebox anyway.
Never driven an X-Type but I have a friend who drives an S type and his Dad has driven Jags for a few years before as well and both have no complaints. I believe they are underneath the stuff we actually see, a Mondeo. I guess it'd be like an ST Mondeo but with a LOT more luxury in it.