INterceptor makes a valid point. Combine a Miata with that cash, and you'll do good. Also, you will learn alot... so its not like you'll be leaning on the car.
With that, you can use the MX-5 for a while, complete an array of races, sell some cars, and get something that is competive at very high levels even, such as an MR2 GT-S (Did the pro turbo race with it) or an Intergra Type R (Pro FF race and All the Type R races, also, got the Acura, so the American make races)
An STi or Evo would also work well after you get some bank. But that 18k into the Miata would make it good for the beginner league at least... well most of it anyhow