1. You can run any size rubber you can fit on stock rims. What year is you Miata?
2. Get the rotors turned and buy new pads. Hawk + or Hawk HPS are good but can be noisy.
3. That helmet would work. Others can be found at http://www.saferacer.com/.
Rules for SCCA http://www.scca.com/documents/Solo%2...lo_Rules_2.pdf