Open Question: New to dirt bikes and dont know where to start..?

I just turned 21 on April 25th and always wanted a dirt bike.. I was told by a Kawasaki dealer that a 450 would do me good justice due to my height and weight (5'11 250 lbs). But another guy who is very experience told me to start with a 250 cause the 450's are really fast.. But some people claim that some 250's are fast also.. I dont think that 125's or 150's will do me justice due to my weight. Can anyone help me out with some good suggestions.. Also, do any race bike comes with keys or do they just kick start??