I'm new to BMW but not new to HPDE, so I can give you some general advice.
the 2 biggest things are tires and brakes.
as you start out, the oem pads should work ok. how fast you need to upgrade them depends on how fast you progress as a driver. if you plan on tracking regularly, stainless brake lines are a must. Eventually you'll want to move to track/race pads. maybe a BBK depending on how serious you are and oem brake capability (I'm new to BMW's so I'm not quite sure how good they are).
for track duty you want to stay away from drilled rotors, they tend to crack and not last as long as slotted or blank rotors. In my opinion blanks are the best. Modern brake pads don't really generate a lot of gas like they used to, thats where the whole drilled/slotted rotor thing came from. Popular misconception is that they cool better/faster, that simply is not true. Some people argue that slotted will keep pads fresh as they have an edge to scrub off potentially glazed pad material. That may or may not be true. If you want slotted rotors go for it, there isn't too much of a down side to them vs blank, just stay away from drilled unless you want to be replacing them often.
while checking out my 328, I noticed it has factory brake ducting (this is awesome!). I'm not sure if its a sport package only thing or if all 3's have them, if you're doesn't, I'd recommend getting them, the ducts are like 30 bucks each (had to replace my passenger side). If you track regularly, you'll want to get some brake duct hose and direct the air into the center of the rotor the best you can as the stock ducts just dump air only into the wheel well, not directly to the rotor. I know for other cars, companies sell brake rotor shields to hook up to brake ducts that will aim the air to the center of the wheel, not sure if something like this exists for the 328 in the aftermarket. The reason you want to direct air to the center of the rotor is cause its vented, it had vanes between the 2 halves. as it turns it pumps air from the center of the rotor to the outside, this is how it cools. if you just aim the air at the rotor face, you'll only be cooling one side of the rotor and be getting uneven pad wear. although if this is the best you can do its better than nothing.
and remember, its not about stopping distance, its about consistent stopping distance between your 2nd and 17th lap.
as far as tires, the two important factors are how it holds up to heat and sidewall stiffness. most street tires will get greasy on you as you get faster. as for sidewall stiffness, you don't want your tires rolling over on you as your cornering, you wanna keep the tread on the road, not the sidewall. In a very extreme case a tire can debead and come off the wheel, although I personally have never seen that unless it was caused by the car going off track.
If you want to stay full street tires, I'd recommend Hankook RS-3, Bridgestone RE71R, or BFGoodridge Rival (non S). Dunlop Star Specs are also good but on heavier cars they can get a bit greasy deep into the session. These are what Tire Rack classifies as Extreme Performance tires. There are more tires in this category, but they're more aimed at autocross where staying sticky for 20 minutes sustained isn't an issue. The RE71R is definitely the fastest (nearly on par with R comp tires). But they're pretty new, so not quite yet proven for sustained performance. Most of what I've been seeing they last well for a full session. The Rival is replaced by the Rival S, which is more aimed at autox, stickier but will get greasy on you. I read in some magazine (Grassroots I think) that BFG is gonna keep making both but they seam to be phasing the original Rival out based on availability I've been noticing on Tire Rack.
If you wanna go full Rcomp, Nitto NT01 or Toyo R888 is where its at. There are other's available, but most are a lot more expensive than these two. But feel free to try them if you wish.
As with brakes, starting out, you'll be fine with regular street tires. My only concern is the V12 is known for its soft sidewall... but you definitely don't need to run out and buy new tires to do a track day. Just once you do get a proper set of tires it'll be the biggest difference you feel on track of any mod... except maybe going from stock brakes to a superbadass BBK.
A must for doing a track day is to make sure you have fresh brake fluid (I recommend Torque RT700, but you should be ok with any DOT 4 fluid) and at least half of your pad material still left on your pads. If your alignment is out of wack, you'll want that done too. And if you're getting realigned anyway, ask them to max out negative camber all around. Just in case BMW's are freaks when it comes to oem adjustability, a good starting off point would be -2.5 front and -2 rear. what you actually settle at depends on your particular car, tires, suspension, driving style, etc. from personal experience, most front engine rwd cars settle at around -3 front and -2.5 rear, but like I said that varies.
For toe settings, oem specs are good for a new driver. As you get better you'll prolly wanna play with that too.
sorry for the long post... I'm bored at work
I actually just bought an E90 328i auto as well, I posted in a different thread asking about how the auto holds up to track use... I've never tracked an auto before, but my main concern is it overheating. Not sure how much of an issue this is specifically for this car.