You asked about turbo. You can't get a turbo for your car.
Supercharger is your only option, but not worth it.
AA = Active Autowerke. These are the things they make specific for 328.
http://store.activeautowerke.com/collections/e9x-328
But honestly, you should focus on cosmetics. Keep it clean and tasteful. Your 16 year old self doesn't need to be going any faster than you can with this thing stock. If I could tell my 16 year old self that 17 years ago, I would. Time and time again. He would thank me now.
Congrats on your car and good luck with whatever you decide.