Currently driving a 2013 C300, which might be representative.
1. Neither COMAND Online nor the Drive Kit Plus work with the iPhone 5 yet, in any MB model. MB needs to update their software, they're late.
2. Bluetooth music / phone / contacts / text messages works fine with the iPhone 5 (non COMAND), in my experience.
3. Mercedes COMAND + iPhone 4/4S Media Interface cable reportedly works fine for music/phone/contacts with an iPhone 5 if you have an Apple 30-pin to Lightning connector. I have not personally tested this.
4. In 2014 there's supposedly "iOS in The Car" coming to MB. Presumably December 2014, at the pace MB develops software.