Try products from Pinnacle, Poorboys, P21S, and Wolfgang. These manufacturers make great polishes and great waxes/sealants. They all work well used by hand, especially the Pinnacle and P21S. The key is to find something you like and use it often and stick with it.
I highly, highly recommend getting a 32oz bottle of Poorboy's Spray & Wipe. It's a great product for any car (daily driver or garage queen). I use it almost everyday on my black bimmer and it has never marred a thing. S&W coupled with two or three high quality microfiber towels makes keeping your Z4 easy, pleasurable, and safe.
Don't use a strong wheel cleaner. BMW wheels are a major pain to care for properly. They are gorgeous when they are clean. They will loose their luster after about a year of using a strong wheel cleaner. Four Star, Pinnacle, Wolfgang, and P21S make awsome wheel cleaners for BMW wheels. My personal favorite is Detailer's Pride Gel Wheel and Tire cleaner. It's as safe and works as well as the others I mentioned but costs considerably less.
I hope this helped. I hope you enjoy you new car, and welcome to DC!!!!