The user registration starter pack includes the registration, login, database, and other functionality you might need.
I would start with that.
Then you could probably the email verification link with the paypal subscribe button and remove the current verification code from the email so that the only way to verify is from paypal.
Then use the current verification page to create an IPN page that will mark the paid user as verified after paypal payment is processed. This would be done with a combination of dataassist and ecart.