Hi there,
There are consistently more people racing in the 17.5 Touring class. The 21.5 Scale Spec class is a little slower and easier to handle for newer racers - especially on our small-ish track but I'd probably recommend you go with the 17.5 just purely based on attendance numbers and the fact you can run the exact same class outdoors in the summer with us. If the motor is too hot to handle initially, you can dial down the power in your ESC or radio.
I don't have any 17.5 motors in stock at my shop at the moment, but could get you one for next week. Let me know if you want me to get you one.
In the meantime, I know we can find you a used loaner 17.5 motor to get started with. If you can make it to the track this week for evening practice, or racing on Saturday, let me know and I'll try to track one down.
PS. I have bodies and tires at my shop.
Thanks,
Blake