Get to know the class before buying a car, your best bet is to buy one already set up and tweak it rather than build one from scratch.
I highly recommend reading the rules. People are running 91s and mid 90s cars for several reasons:
Spec Miata rules PDF
The normal races are sprint races and you wouldn't have any pitstops...that would only become important if you ran enduros. The FIRST thing to do is go to a couple of race weekends and talk to people and get a feel for what's going on before considering spending any money.