20k is a lot for a sport bike, unless you get a real good deal and don't mind replacing some parts like clutch, chain, sprockets, ect. also at your height and weight a 600 would be perfect. don't waste your time on anything smaller or you will be selling it in 3 monthes...

it would be a real good idea to go to a bike shop and sit on a few different models and see whats confertible to you