| Hello there.
The W/S mode button stands for Winter and Standard. Winter mode increases torque and thus adherence when starting. I believe that under normal conditions, standard mode is to be prefered as it improves acceleration and makes the ride more comfortable.
The ACR button inhibits the electronic drive assistance. This again might be necessary when driving under certain winter-conditions (snow, ice), but should normally be let active.
I can't remember what the EC button is for, but if you really need it I'll have a look in my manual.
I hope this reply brings an answer to at least 2 of your questions.
Last: the owner manual brings a lot of detailed information regarding the use of the car. For instance how to use the sequential transmission mode, accelerator kickdown, how to manually open the fuel reservoir, aso. I therefore recommend that you go to your Mercedes Benz dealer and ask for a copy. It isn't a huge investment, and it's well worth the money.
Regards. | 
