07 KX450F ???

it would be difficult to say how quick it is without knowing the gearing its got on it, but that said your basically looking at about 90-100 mph. The kwackers are great bikes nothing really that you should look for specifically just usual stuff like has it been well maintained etc. A clean bike usually indicates good maintenance but just give it the once over checking bearings etc and most importantly the motor, check with your hand to see if it is warm before you start it because if they run it before you see it there may be a problem. Check it sounds ok and go through the gears just to make sure its all ok.