i think a 600 is the best option of those that you listed. they are forgiving bikes in several areas to the mistakes that you will make (repair costs, wreck severetity, etc). you can run almost every race that a novice possibly can on a 600 which gives you more seat time. more racers seem to be on 600's which make available spares easier.

and i would deffinately start out with a used racebike as a learner. i started out wanting to make my streetbike a racebike and was convinced by some experienced racers how bad an idea that is. then i tried to build one from scratch, which is not easy for a first time experience. finally, i bit the bullet and bought a used racebike. finally, i started having fun and actually started to do better, even put myself on the podium in the novice gto.

whatever you decide to do, good luck and you will find this group to be a great support in your efforts.
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Fourth-generation Mustang