If you happen to go look at a 90 or an early 91 Miata, I suggest reading this as well: http://www.miata.net/garage/crankshaft.html
I will also add that an automatic Miata is a bad idea. It completely removes the fun of the car, and yes, I have driven one to confirm. It's an option that Mazda should NEVER have offered for the car (RX7 included). Good luck with your search. If you really want one, I suggest waiting it out for the one you want. They are fantastic cars.