Hey Bethany,
I'm biased towards the '94-'97 Miata myself. They're cheap and way fun to drive and horse power is overrated.
I am also biased towards getting a fun autocross car so that you feel like if you know how to drive, you'll be competive. I know lots people say, "Develop the driver first" but how do you know if you're developing if you're not in somewhat the same type of car as the people you're aspiring to drive like? Plus, when the car's set up right, it's just more fun. So if you can swing it, go for it.
Before I tell you what you need (I just skimmed all the posts, so I don't know if this was covered), you might also consider co-driving with someone for a year (or even forever!). You're basically renting a ride, which in some ways can seem even more costly, but if he or she's a good driver, you may learn a thing or two in hopefully a supportive environment. The way I found a co-driver is I'd show up at events with a helmet and waved at people, and that usually did the trick. I had a few points in my favor, though, like I grew up autocrossing and everyone knew who I was, and who my parents were, and I guess people just felt bad for me, I don't know, but I landed me some sweet rides. You might also try your local e-mail list, and just send out a note saying you're looking for a ride, maybe for the season, and ask how much people would charge for such a thing.
Nothing was sweeter than driving my own autocross car, though, and I am thoroughly supportive of all women who go this route. It's not the hardest thing in the world and you will have a BLAST.
If you do get the Miata (or whatever), you'll need, eventually, in no particular order of importance: a portable jack; a tire pump that you plug into your cigerrette lighter; tire pressure gauge; a torque wrench; one of those big star wrench thingies (women know what I'm talking about while men will only understand the more accurate, technical term, and I have no idea what that is); some kind of knee pad for changing tires; an ability to change tires or at least a desire to learn; an extra set of rims with your race tires (I should have mentioned this part first, and I'm way out of the loop so I have no idea what the set up is); a set of magnet numbers; some kind of cardboard storage thingy for hauling your magnet numbers (plus all those other necessary magnet decals); that duster wipee offee thingy that uses static cling; special racing seat belts (maybe not that necessary, but makes you feel fast); fast sounding exhaust; after market shocks (Koni?); a big, big front sway bar (unless you do the front wheel drive option, and there I cannot help you); a special autocross alignmnent; gardening gloves (should have mentioned this earlier) and/or latex gloves to keep the hands clean and also for wiping off tires between runs; baby wipes for post changing tires clean up.
Well, that's off the top of my head. I hope this helps!
Katie Kelly
Retired Autocrosser