why limit yourself, there are some excellent alternatives out there.
How about a volvo S40 (or C70 if you are ok with a petrol) or a Pug 406 HDi (excellent car for the money - ask don shiftini) you could also look into a Saab 9-3 or a honda accord (if you can find the extra £££ - they do fetch good money). the Alfa 159 is excellent but will still fall well outside your budget, as they hold their money like crazy, and still haven't really dipped below £7k for a high miler. the 156 is getting a little old now, but a late model could be had with the 1.9 JTD for about £4k - buy carefully and they are very good cars.
Passats and A4's are ok, not exceptional, and fetch a lot more money than they are really worth.
I'd avoid the Vectra like the plague, aside from being unbearably dull, they have a lot of problems. I'd not jump for joy at an X type either, unles its the 2.5 or 3.0 neither of which are going to be any more economical than the stang (although they will be a lot quicker) the MK3 mondeo is ok, but no better than any other alternative.
whatever you go for, check the DMF carefully, on almost any car they are a £1000 repair when (not if) they fail.