I do engine oil every 5,000 miles with Pennzoil Ultra Platinum and an OEM Mazda filter (2 stripes!)
Hitting 30k now, I also did a drain and fill on the rear end, transfer case, and transmission.
For the tranny, use Mazda Type FZ for the ATF, took about 4 quarts, and for the correct level I...
Sucks you missed the RX-8 wheels, they look SO good. But yup, those MZ3 wheels will also work! If you can find some, Mazdaspeed3 wheels look really good, too.
The 215/45R18's will be slightly taller. It'll work, but it's not perfect. If you get those, obviously you'll eventually have to change tires. When that time finally rolls around (get it, rolls around, lol), go to the 225/40's.