3 - I bought a good pair off Ebay... but other sites sell them too. Be very very careful when buy stuff off ebay. Best to stick to GOOD and KNOWN online sellers.
5 - You can do a direct plug and play replacement of your BMW deck with a BMW deck that plays MP3s. The AUX option is good. I recently purchased/installed a MediaBridge and like it so far...
6 - Man, I can TRIPLE CO-SIGN on this...
I've had my car for 2 years and I've had the following:
Expansion tank explosion.
Clogged vents! (stupid me drove in Snowmaggedon 2 years...)
Power steering leak. (simple seal)
Front shocks leaking. (Ironically, got damaged driving on the 427 and running over someone else's front shock! SOB!)
and a few others...
Depending on where you go, will depend how much you gotta "blow"...
I personally like RMP, they are pretty straight forward with pricing...
The best thing is to LEARN as MUCH AS YOU CAN about your car.
Don't be ignorant.
And if an IDIOT LIGHT shows, check it out.
I love my car, but I can see why it would turn other people off.