You'll be able to get a bigger machine within your budget if you convert it yourself, unless you get very lucky.

I got my used milling machine for £320 including delivery via eBay. It's this one:

http://www.machinemart.co.uk/shop/pr...C=GS-060711226

Have a look on eBay and sort by distance nearest. You'll want to replace the screws with ballscrews from linearmotionbearings2008 on eBay. Electronics will cost about £250.