would the 300 be too much bike for me and too big a jump from a kx 125??
You will learn to control and love the power. The 300 will lug down more than to 250, the 200 you will need to be in the pipe like a 125 to get the power needed. I forget your weight, height and age.
26,riding a few months,12 stone,6ft 2 inches
My opinion would be the 300 is your best choice, next would be the 200. Go look at all three and choose which one looks the like the most taken care of. They are all priced about right.
200 is hard to beat.. but they are all good