Purchase a 540 ?

I am a 540 / M5 admirer, but not an owner yet.

I currently have a friend looking for 1999 or newer 540s in southern California (because I know BMW sells more there than anywhere else in the world).

I will take any advice you folks have to offer. I am aware that the Bose system seems to be a necessary evil, but that's all I know to worry about. Any particular year advantages/disadvantages?

Thanks in advance,

dafried