Getting a new bike need a little help

Ok here is the deal im getting ready to buy a new bike but im not sure exactly what to get and im sure this question has been asked a million times to bear with me. First my experience I have been riding a street bike (gsxr600) for about 2 years now and have rode dirt bikes for about 6 months. Im about 180 and 5'11 i am mostly going to be in the woods but am going to start to learn how to race. I am thinking about getting a Yamaha 250 but im not sure if i want a 2 stroke or 4 stroke? Also Im not sure if a 250 would be to much although i dont want to outgrow something quickly? So suggestions are greatly appreciated.

Im going to purchase a new bike I want to start trying a little motocross I have always been interested and regret not getting into it b4 now. I think I have made up my mind on getting a yz250 I know it is a little much for wooded area but I think I can manage.Now I just have to learn how to do the engine work being the top end will need to rebuilt at some point.