Hi Laura, Bike sizing is a personal thing. It depends on several things, such as your leg length, arm length, torso length, and as everyone is different bikes will be the same. if you could up your budget by a hundred or so there maybe a few bikes that would do initially, and then you could upgrade various parts of the bike as they wear out. Best talk to your local bike shop. Stay away from Halfords.
Maybe somethig like this?
http://www.evanscycles.com/products/dawes/giro-300-2009-road-bike-ec018618
Maybe somethig like this?
http://www.evanscycles.com/products/dawes/giro-300-2009-road-bike-ec018618