Welcome to GRM and the joy that is Miata! I went through what you're doing now a few years back and here's a few completely random tidbits which may or may not be helpful.
Koni Sports (or yellows) and tires are pretty much the only thing you need to have a blast in your car. I'm a big fan of the "long-bolt" method to install the shocks, which basically means removing the upper control arm bolt and using a jack to gently unload the suspension while you swing it down and out. No matter what you might think, you can never have too much PB Blaster and fire when doing this job. You want spring compressors and you want new bump stops (and other rubber bits) ready to go. I wouldn't worry about bushings unless there's severe slop, but this would be the time to install them (in which case, screw the long-bolt method; you're disassembling it completely!).
After you've gone to a few events using whatever you've got, think about tires. For SCCA street class (the new stock), pretty much the best bang for your buck is the Dunlop ZII. They'll last you a year's worth of racing and road trips and are well-suited for a crisp driving style. As a bonus, they're one of the best 200TW rain tires!
Regarding the front sway bar, it's a nice part to upgrade (especially AFTER a few events so you feel the difference) but if you do, get reinforcement blocks to prevent your stock bracket from breaking. It's a pain in the butt to replace and if you're an idiot like me, you'll get stuck on the 5 bolts you broke off in your frame rail and wonder how to get rid of spot welds. These support blocks are drastically cheaper and easier to install than the Mazdaspeed/AWR replacement bracket. Don't let this happen to you!
For brakes, don't get HP+ pads all around. Stock Miatas put too much bias up front. Cheapies in the front, HP+ in the rear is great and mostly lockup free without ABS. Advance Auto sells caliper hardware kits for $10 if you're missing springs and miscellaneous doodads.
Don't get a rollbar unless you're going to track the car. Safety is a slippery slope and before you know it, you'll need a harness bar too (unless you got a Hard Dog like a smart person), racing seats, harnesses, and a Hans device. At that point, you're in STR or STS, so you might as well get wider wheels, new tires again, a custom exhaust, some weird engine management that's legalish depending on who reads the rules, and you're broke now why are you still reading this. A foamectomy is not a substitute for a racing seat.
Don't bother buying replacement OEM floor mats (assuming you need replacement, that is). Just get the rubber ones Moss or whoever online sells. They're much nicer, don't mess with the clutch, and won't leave you feeling sad when it rains at your autocross and they're out of your car getting drenched.
Get new NGK spark plug wires once every 2 years or so. Mobil 1 0W30 is good stuff. A Mazda Millenia oil filter fits on our cars and is longer, which helps you contort and remove the darn thing. If you have a NA, you want lots of Nyan-cat decals and a vanity plate that says POPTART.
That sums it up...