In answer to your questions:
1) The default payment gateway is PayPal, and you can purchase
other payment gateway plugins from the AppThemes marketplace, however this does not currently include one for Stripe which would require customization on your part if you were looking to integrate it.
2) There is no setting for this within the theme, and whilst there are plugins which are made for this purpose such as
Network Privacy, it hasn't been specifically tested with this theme so we cannot guarantee it definitely works out of the box at this time. AppThemes does not officially support third party plugins.
3) Through customization it can be done, but how "easy" this is, is relative to your level of coding knowledge. We do have a
certified partners page if you are seeking a developer for hire who is experienced at working with our themes. Thanks.