I think this would require more of details consulting than some quick answers.
1. On an highlevel you can do this project on 80% OOB functionalities from CRM with Accounts/Contacts and Service module case Management system in cobination with custom entities.
2. In terms of the security, CRM 2011 supports Form based, Roll based, field based, object based security which can be used to specifically target your commmunity needs.