What Mixer should I buy?

You will probably not find an increase in quality in that price range, save your money and get an interface, some decent preamps and start with that
 
Best bet at that kinda price would be a Soundcraft EPM which has GB30 mic pre's (the same that are in the LX7 and GB series mixers)

There's also the Allen & Heath Zed 14 for about £300. Or you could pick up a second hand Mackie Onyx mixer for about £250.