Good Luck with your hunt and purchase. Recently ordered one...waiting for it to show up at the dealer. "Must have" options I guess are very subjective: for me they were:

1.Spare: I plan to ditch the RFTs the first time I need to change out the tires...
2. Comfort Keyless entry: used to it with my present cars..did not want to do otherwise
3. Premium sound: dont know how true:but heard not so good things about the standard audio system.Also there is Siruis for 1yr included.
4. Driver assistance: heads up display is waaaaay toooo coooool to pass up
5.BMW apps: if u use iPhone, ianything

I did not choose rear entertainment: have more than one child. Like the dealer told me: for the price of the single rear entertainment system could get 3 iPads....
Did not choose the 6CD changer as its all digital music these days...

Hope this helps. Again good luck with your search