Magento extensions

Background. I require an extension to work with Magento v 1.6.1, ideally without affecting the core files so we can upgrade the site in future. The website in question sells video courses that are sold for 30/60/90 days subscriptions. At the moment there are less than 10 courses or individual downloadable products on offer. Each downloadable product has a bunch of files and a CMS page URL associated with it for that course. Requirement. [login to view URL] 1 or more customer groups for individual courses/products 2. When a customer purchases a course, s/he is automatically assigned to that specific group based on the course that was bought. One customer can be assigned to multiple groups if a customer were to purchase more than 1 course. 3. Each course has a valid subscription time e.g 30/60/90 days which needs to be set individually for each course. So once a customer is assigned to a group, s/he has access to that course for the subscription time period after which they are automatically set to the default retail customer group. 4. Assign 1 or more customer groups for individual CMS pages. 5. CMS pages need to have restrictions applied based on the customer group, so that a customer can only access pages that are associated with a certain course. Again this restriction will be time based (same as number 3 above) so once the subscription is over, the customer can no longer access the CMS pages.