Hey,
As I have been in your situation 2 years ago, I can help with some advice

.
1. European model cars (both CAS or non-CAS) can open the roof with the key by holding unlock, the closing feature is only available for the CAS tough. For the US market, only the models that have CAS can open and close the roof with the key. If you want to retrofit this to your Z4 you have two options:
SmartTop that offers more functions (for example folding mirrors on holding lock.. etc) or a little cheaper, german alternative that only offers open/close roof via remote (I am missing the link for this one).
2. Little more throathy exhaust - I recommend remus exhaust
3. At one point or another you will have electrical, then hydraulic lines snap on your roof.. this will happen to all Z4's and you will probably need to change them at some point in them when you encounter a red flashing sign on your dash. What I'd like to tell is - if one sensor is not working on the roof and you started to open/close the roof your car will get stuck in that position, not allowing you to access the trunk or operate the roof. If you lack the knowledge and tools to put it back into position it will stay as it is - for example, until I've changed my hydraulic lines on my z4, i've never opened the roof when I was away from my home city.
4. I've never heard anyone retrofitting bluetooth on a Z4 E89 but it might have been done, just look around on this forum, it has a lot of resources.
5. Possible upgrades:
-
Front brakes upgrade (since a lot of people started talking about them on the forum)
-
Cruise control retrofit
- Android screen instead of the iDrive (you can also retrofit it if your car did not have iDrive), you can find them on aliexpress for about 700$, pretty straightforward to do
- OEM Winddeflector retrofit (can be done for around 3-400$, not that hard either)