I just bought a car a month ago and I want to upgrade my head unit.

Right now, my head unit just a plain cd receiver. I would like to put navigator and also be able to play mp3 music while I can still use the control button at the steering wheel.

What is my choice? Can I just replace it with after market head unit? or do I have to replace it with Mercedes head unit only to keep the button at steering wheel work?

Thanks for helping!!!