Like everyone else said, either an 85 or the 150R. They both have pros and cons but either one would fit you just right. The two stroke 85's are cheaper to buy and cheaper to maintain. They might be a little harder to learn on because of the power delivery.
The four stroke 150R is expensive to buy new and used and is a higher maintenance bike. You don't want to skip on regular maintenance or you will be paying some big bucks later down the line when something goes wrong. But, the 150 will have smoother power and is easier to ride on tracks as well as trails than the two stroke would be.
Also, just remember, depending on local rules for racing in your area, you might not be able to race the 150 in the 85 class.