You need to identify what class you want to be in. If you go with a '90 & up Miata, that is going to put you in CSP if you need to change the seat.
That can be a fun class with a lot of competition. You'll be up against 944s, Civics, Miatas, MR-2s...
SCCA Solo II Car Classifications: Stret Prepared by Class
From that list, I would definitely go with a Miata. Most bang for the buck.