trans am or camaro v8 lt1 or ls1 depending on the year.
they are very fast and can get faster
you would want a "f body" after 1993.
production ceased on these cars in 2002 i believe.
scion tc isn't really a sports car, and you would be hard pressed to find an rx8 for 10k. also with the rx8, upgrades and performance is limited compared to imports (turbo kits etc) and domestics (cam shafts, throttle bodies etc)
the rx8 has a rotary engine, which I'm sure you're not familiar with, and would probably not be looking to get involved with as your first car if you could simply get a nice 350 ci small block v8 to learn and play with. i kno the t.a and camaro are older, but they are fast as hell and still look great
for the record, i picked up a 76 corvette, which was one of the least desirable years for their engines, and upgraded the whole thing, for under 10k, and i have something i can bring to car shows. if you can get lucky, and find a classic, more power to you. literally.
drive safe.