Of course, the answer is always "Miata", but I still say run a season or so in the Fusion. Get seat time and see what other folks run in your region. Yeah, I'm sure running a Prepared car is fun, but not if it's the only car in the class... To me, it's more fun to run in a class with a few other competitors... even if it's the slow-azz HS class I run in. It's not just the competition, but also the commaraderie of the event. At least for me. I have little tolerance for those who take it too seriously.
It's easy to go nuts, tho... for example, one of our region members sold his SM class E30 and bought a Miata to run in ES-class ... only he's now in the process of turning the car into an R-spec car... and by the time he's done, will probably have more $ into the car than he paid for it... Granted, he's not just going for class wins but PAX as well (where his E30 was heavily handicapped)... and already has a few years of auto-x experience...
Basically, the nice thing about running in stock class when starting out is it more or less takes the car and mods out of the equation. You just concentrate on driving correctly. A car that is too good can mask driving errors.