Hi,
1. Well, you can add custom PHP to the view displayed when you arrive in your shop. As we don't know what page you're using for that, we can't tell you where you should add that exactly.
2. If you want to customize the display of the add to cart button, you can edit the file "quantity" of the view "product" via the menu Display>Views.
You should find there several times the line:
echo $this->cart->displayButton(JText::_('ADD_TO_CART'), 'add', $this->params, $url, $this->ajax, '', $max, $min);
which displays the add to cart button. So just add a condition on that based on your subscription.