Dude, slow down. Like the folks said, your battery is your first target, and if it's toast then that can give you a whole variety of symptoms that are not typical of your past experiences with a bad battery, and may not be consistent from one episode to the next.

The reason is virtually all functions are controlled by various modules, and the modules communicate with each other. Without going into detail, only a couple modules control; the others obey and run their little "departments." The communications is not at all high tech; it's basically the same as a 30 year old fax machine. But when the battery is flakey, then signal levels fluctuate or can go below minimum levels necessary for reliable communication between subsystems. Different systems will fall off the grid at different levels, and that can vary depending on specifics to that situation. The gotcha is that upon startup, there's a lot of handshaking going on as modules "roger up" with their status - and this is the same period of time when the alternator hasn't been brought fully online (depending on model, the system brings the alternator up (field current) after a short period of time so is not loading down the motor right at startup). And you have a perfect little storm, and what looks like all kinds of stuff going on.

First make sure you have a good battery; an alternator check is super simple, but don't go off in different directions - you hear hoofbeats, so ponder horses here.

Like someone said, a compatible battery in size and capacity can be had from a typical chain store. There's no magic benefit in spending considerably more to have one with a BMW sticker on it.