“I know that to perform a paypal payment, an user should have a paypal account” – this statement is not entirely true. If you don’t have PayPal you can still make a purchase with credit card.
Anyway, you should be able to achieve what you are after very easily. Search for the following function in the “eStore_misc_functions.php” file:
function get_button_code_for_product($id)
You can introduce your “is_user_loggged_in” check at the beginning of this function so if the user is not logged in you can show the “Log in” link rather than the button.