1. Customers of a certain groups (of customers) will be able to purchase a maximum amount of X items from a specific product ID.
2. User should see an appropriate message in front-end when he tries to buy more the X amount.
3. The configuration for the whole feature should be available in admin panel (customers groups, the amount X, the product, the message the customer will see etc`). I think the right place to put it will be in the product page in the admin panel.
4. Everything should be per store view.