I'm toying with the idea of getting one of these, they are quite reasonably priced used, and I have just been quoted £550 to insure one.
Any advice on the model to go for. Luxury, Premium Luxury etc., and model year? I'm looking to spend up to £25k.
Any known issues with particular models?
PS My low mileage 3.2 Elite will then be surplus to requirements. 
£550!! At your age?!!

That's horrendous.

As with all modern cars there's a lot of silly things. Touch sensitive interior light for instance that becomes posessed. Replaced under warranty in mine by the dealer. Cost them ~£350.
Handbrake needs resetting via computer if you replace rear pads etc...
Typically Each step up will cost another £3-4k - Lux/Prem/Port.
Suspension is very firm compared to an Omega. Even worse on 20"s. Personally I find them quite boring and figedety to drive at less than 90mph.
The touchscreen control is slow and dim witted and everything is accessed through it.
6spd gearbox infinitely preferable to the shitty 8spd variant. Start stop cannot be permanently disabled in the ECU according to Jaguar tech and will reactivate every time you turn it on.