Hi,
Yes it's possible.
You can restrict the display of the products to a specific group, so for example only for registered users.
PayPal is one of our available payment method.
On the product edition page, you can attach files, the not free ones will be available only once the product has been purchased.
So ok for point 4a.
Finally, the vote / comment system can be used and restricted to the users who bought the products.
If you want a review via email, you can potentially use the "question form" on the product page, change the translations and hide the button to specific users.