Unlimited is a 4 door Wrangler.
Not terribly familiar with the UK versions, here in the US we have Three trim levels on Wranglers which I think applies worldwide. Some of the ones you mentioned are limited editions, like the Call of Duty models.
X: this is the base model, power windows, air, alloys, etc are optional. Most of them have power windows, locks, air, etc.
Sahara: this is the luxury version. We get lots of them with Navi, Heated leather seats, etc. Sahara trim means body colored fenders. Sorry, coloured wings
Rubicon: this is the one you want if you're going off road. Updated suspension, the all important Dana 44 axles, locking rear diff, etc. you can get a Rubicon with Nav and other luxuries as well.
I assume you're in the UK so you'll probably be looking at a Diesel, we don't get Diesel Wranglers here, sadly. As far as buying goes, the 2011s got the MUCH nicer dash, and the 2012 V6 Petrol versions got the dramatically better "Pentastar" V6.
But if you're in America buying a used one makes little sense, they have the resale value of gold so you might as well go new, what I pay for used Wranglers at auction is scary.
Edited to add the V6 is worth looking into if you can swing a 2012, otherwise personally I'd get a Diesel of you can. If you can swing a 2012 the Pentastar V6 is powerful and no worse on fuel than the previous engine, which I'm not a fan of.