Well, you can never go wrong if you keep a BMW stock.
I kept my car stock. Although I must admit, sometimes I want to change some stuff here and there. Money-wise, it's just not worth it, unless you're trying to impress people. Even then women will think you're selfish if you put too much money/mods into a car, haha.
If anything, put money into tinted windows (35% is good) or replace parts that need replacement.