Sounds like you will be happy with a hardtail.You can get a decent HT for 600.On the disk brakes, look fo avid mech.,I dont thingk your gonna find a bike with hydro's in that price range.As far as brands try Specialized,Iron horse,Kona,Trek and Giant. Those are a few but there are a lot of good bikes out if you look around.Hope this gets ya' started,
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Like a circus monkey on a stolen Harley......