You don't have to stick with one manufacturer brand at all if you don't want to. For instance I have an Alpine head unit, Arc Audio Amplifiers and DLS cabin speakers and a JL Audio subwoofer. Some of the 'best' manufacturers of each part of the audio setup, might only make that product - HAT for example only make speakers, DLS and Hertz for instance make excellent speakers and also Amplifiers.
I see you've started multiple threads, I'd suggest perhaps reading up some more on specific car audio forums to learn more as well as across the various BMW forums too. Speak to local reputable car audio installers/dealers as well, and then you can make more of an informed choice of whichever direction it is you wish to go.
If you were anywhere near West London, UK, you'd be more than welcome to have a listen to my setup!