if your gonaa get a bike for speed, then get a street bike... its stupid to get a dirt bike for speed... even in racing, your top speed doesnt matter...
i have ktm 200 exc, and my dad recently got a ktm 525 exc... and it does 110 mph... but when it comes to trailing i'd take the 200 any day over the 525... the 525 is faster but i dont need it in trailing...
when you mature a little bit, ask another question about what bike you should get...