Oh man, you should totally go for it! How old are you anyway? If you really are 50+ you should get out on the track before you keel over and die.
I'm getting my friend who's 64 back into racing with karting. He couldn't be more excited.
Luckily, I wont have to pay much because I will be racing his karts that he now tells me I have part ownership over.
Spec Miata sounds like lots of fun, light RWD open top racing can never be bad!