Dan, you have a car that's already built. Forget the Miata, forget spending a lot of money on the Escort. You've also mentioned that your budget plays a factor. So, work on improving your driving! You don't have a competitive car, which can honestly play in your favor since you shouldn't be as tempted to keep developing it. After you've improved your abilities, then start looking at the car.

I look back at how my car was classed in ITA where it had absolutely no shot and realize that wasn't the worst thing that happened to me. AFTER I had some time to improve the driver, I was then able to start improving the car. The driver should come first.