Welcome to the forum,
For that sort of budget, you could get something really really nice on here...
What you'll need to do tho is roughly work out what sort of era you're looking to buy from, and concentrate on that. For example, later bikes from the mid-late 90's will have V-brakes/disc brakes, suspension and more gears.
And older stuff from earlier 90's and late 80's will have little or no suspension, nicer frame materials (IMO) canti or even U-brakes and mild to wild (inc. fluorescent) paint jobs..