In answer to your questions:
1) There is no functionality built into the theme which would prevent posting based on user email address as you describe, so this would require customization of the default theme on your part.
2) There is an option to allow reCaptcha on the registration form, but not in other areas of the site. It can technically be added with customization.
3) ClassiPress is not encrypted, so there is nothing preventing you from making modifications to the theme to suit your needs. The code is clean and well commented, but ultimately how "easy" it is, is relative to your level of coding knowledge. Thanks.