I used to do track days in a Miata, and will once again I make more money.

If you want to do open track stuff, you'll have to get a roll bar for most tracks. Especially if you do organized track days with SCCA or NASA. Its legal for all SCCA Solo events as it is a driver safety device and it will add to the stiffness of the chassis.

Porterfield R4S are good dual purpose pads for street and track. Also upgrade your brake fluid and lines. ATE Super Blue is a good track fluid.
If you're going to get real serious have two sets of pads and rotors. Track only pads are noisy and don't stop well when cold.

But the best money you can spend for improving your track times isn't your car but getting some coaching.

Oh and the stickiest tires you can afford doesn't hurt. Again a street set and a track set will make life more comfortable.