Well, its hard to give advise for the bike you want without much info.
What kind of riding are you going to be doing? I ride a KX 125, and also have an XR 80, and when I go trail riding, I bring the XR 80 out and can keep up with all the 2-stroke 125s that come riding with us, since we ride tight and technical trails. Owning a 125, I will tell you that if you are mainly trail riding and riding in tight areas, a 125 is hard to handle.
I agree that even though you ride a quad, you will still probably have the same learning to do as anyone would. But you will have a jump start on the basic way to control a vehicle and such.
Maybe a KDX 200 would be good for you.