1. Unfortunately, the ClassiPress team does not provide any custom services at this time.. If you refer to our resources page under "Custom Development" this explains more about recommended alternatives..
http://appthemes.com/resources/
2. Yes, WordPress plugins can be used with the ClassiPress Theme, but obviously there are those which are recommended and tested plugins, and anything else is really up to the individual to test.
3. Yes, you can install ClassiPress on an existing host account.
4. We don't provide plugins as such, but again if you refer to the resources page you'll see the recommended plugins. I know there are current ClassiPress users who are using the star rating plugin successfully.