I believe most of the hardware is similar to the RX-7. Having worked on 40+ RX-7s, parted out a pile of them, built 20-some motors, I'm pretty familiar with what sizes are on the car
8mm - some minor trim. Not too common.
10mm - LOTS of stuff. Most every bolt in the interior is 10mm, many brackets in the engine bay are held on with 10mm bolts, etc.
12mm - again, LOTS of stuff. A lot of the engine accessories are held on with 12mm fasteners - intake manifolds, AC compressor, water pump, etc. Very few things interior-wise are 12mm.
14mm - Again, mostly under the hood. Tranny to engine bolts, starter bolts, etc.
17mm - You see this a lot in suspension, and I think the various exhaust flanges have 17mm nuts.
19mm - Not super common, but a lot of suspension stuff. The RX-7's front pulley was a 19mm bolt.
21mm - Lug nuts, some suspension components.
If you have 8, 10, 12, 14, 17, 19, and 21mm tools, you can tackle 85% of what's on the car. The oil cooler lines and AC lines on the RX-7 required 21mm, 23mm, and 26mm combination wrenches - not sure about the RX-8. The main nut that holds the flywheel on is 54mm - no kidding
. You can also use a 2 1/8" socket for that. I wouldn't buy one unless you were fixin' to remove the flywheel - no need.
Get some good combination wrenches, regular and deep well sockets of each size, and you're good to go. Many times with Sears you need to buy a set to save a little money, but you end up with a bunch of tools you won't need (15mm, 16mm, etc.).
Have fun tool shopping!