is your budget for 10k for the car itself? or 10k including tires, brake pads, misc maintenance etc..?
put reliability above all out performance, tracking a car brings out so many more issues, getting some modded RX7 will just leave you with a broken car and half a track day.
you want something NA, will be much more reliable and trouble free than a turbo, and especially dont do some aftermarket turbo kit on an NA car.
you said you were looking at an RX8, thats not a bad car if you have extra money to prep it and use for maintenance, they are extremely strong engines, especially in a track/racing environment, they only get a bad rep because people dont realize how much oil they consume and treat it like any other normal car. if you keep on top of it with oil changes and the few little quirks that come with rotaries, you will have an extremely solid car, but expect to spend 15k before you are "track ready"
personally id go with either a miata or an e30 325, both have a great reputation for being tough, good performing entry level track cars. and they both have a ton of support parts wise, especially the miata, you can find parts extremely cheap, its easy to work on, people are tracking cars with 250,000 miles on the engine, the things are little tanks.
they have by far the most support for aftermarket/track parts, and you can always find people at track days or wherever who can help, there's tons of info on setup's and how to make a miata fast.
get a miata with a lightly modded engine, good track suspension, roll bar, lightweight race wheels, lightweight flywheel, maybe some body stiffening, and slicks and you will be outrunning guys in STI's and much more expensive cars.