HI Moritz,
I am trying to do the same thing as you.
to make the shop page and all products visible only to registered users.
i had a look at the S2Members plugin you mention, but I don't think I understand what you mean by
"However: the shop-page itself will remain visible(accesible) even if I managed to get it out of the menus"
so, does the plugin protect the access to the SHOP page and all products or not?
Ideally what I want is for the SHOP page to be visible on the Menu, but after clicking on it, I want the page to have info informing that this is a members only page and have a login/registration area.
I understand you have the login area on the side bar, right?