When it comes to outfitting the car, you might as well go ahead and make the Miata be Spec Miata-compliant for NASA/SCCA. It'll be easier and certainly increase the value of the car since Spec Miata is one of the more popular classes. Besides, if you end up deciding to race, you will already have a car ready to go. (BTW, that means a window net and not arm restraints because Spec Miata requires a hard top.)
As far as your personal safety gear, you'll need a suit rated for 5.2 or better I think. Most people just go with the single suit and skip the underwear. Facial hair does require a nomex hood and you'll want socks too.
A good palce for reasonable prices when you are starting out is Racer Wholesale.
http://racerwholesale.com/