[Billy10mm]

Some other people have already alluded to this, I'll be more direct: Stop worrying about what to buy. It's irrelevant. You're going to be slow. Painfully slow. Your tires, wheels, and brakes will not be what hold you back. The track is going to be expensive enough without you draining your account with needless upgrades.

Get to the track. Find a track event. Check your local BMW CCA (which does not require BMW ownership to join or participate), all local track-day clubs, and if you can't find much - then go to a local auto-cross and ask the people there, quite a few will know of all the groups you can't find in the Googles.

Track for a year, maybe two. Go to as many events as you can afford (I budget between $1K and $1.2K per track event ... and its likely more than that if I add up everything). As you get faster, as you talk to people, as you learn what it means to actually get faster, you'll start to realize what the limiting factor of your car is.

We can start with the fact that you have a convertible with no roll hoops. That's likely going to kill you up front, but you might be able to find an organization that allows that. Once you get fast, you're going to bump up against brake fade. The fix won't be cheap, but it will save the car (and you), so do it right.

Then will be the turn-in, and you can address suspension, alignment, and rubber as needed and as funds make them self available.

Seriously. The "what do I need to buy before my first ever track day" is a face-palm moment for those of us that have been round a few corners. It will be thousands of laps before you start needing upgrades on a 400+ horsepower pony car. Work on the driver part for now (and for a long time to come).