I've has bikes since I was 12 and passed my bike test before my car test at 17, I've done the usual Jap sports bikes RGV250,NC30,ZX6R,GSXR's and currently have a Ducati 900 Monster with some mild modding, brakes, suspension, exhausts etc and its a right grin if not a little loud it, sets car alarms off
My advice would be take it steady to start with and get somethin on the sensible side, the modern 600 sports bikes are mental fast really 110ish bhp and 170ish kgs and 170mph isn't really novice tackle, fcuk me I sound old, sayin that if I was 20 again I would be probably be goin out and buyin the fastest thing I could afford, in fact thats exactly what I did