In answer to your questions:
1) ClassiPress is a WordPress theme, so you need an instance of WordPress installed in order to use it. It is not a plugin, so cannot be installed onto an existing theme.
2) As outlined in the
FAQ:
What are the requirements to use AppThemes?
AppThemes does not work as a standalone solution. It is a theme which runs on the free CMS blogging software, WordPress which requires a web server such as apache, a mySQL database, and PHP. If your hosting provider allows WordPress to run on it (over 95% of them do), then AppThemes should work fine. You might also need FTP access to upload the theme and admin permissions to change/create folders. This is very common for any web hosting provider. We are partners with some of the best affordable and reliable
web hosting providers out there today. They are certified WordPress partners and will have no problem running our themes.
3) Only verified AppThemes customers can access the forums, however you are free to post pre-sales questions here without registration as you have done. Thanks.