Why not look into getting a lotus exige? Look for the first production year of the Exige S as they offered a non-supercharged motor (I believe it was 2005). It's a toyota 1.8L so it's damn reliable and makes 190hp. Also only weighs 2000lbs. That's going to be relatively cheap to run as well on the track and it will be far faster than a miata. Then after a few years, you can always add an aftermarket supercharger. Instead of only bumping you up to 220hp like the factory supercharger, you can go to 240-260whp.

--L.D.