I have just bought this car used and now this light comes on , is this something major or just another reason to take to dealer

 

its the braking system for the vehicle, BAS is Brake Assist Program and I don't think Mercedes has any "ARS". Do you mean ASR, Automatic Slip COntrol or ABS, Anti-lock Brake System?
All these three systems are related to the brake system in one way or another, so its best for you to have a dealer or any auto shop with the Mercedes STAR system to chcek it out. But most of the its just a faulty sensor.