1. Anything’s possible. You’ll just need to build it. BuddyPress doesn’t handle authentication, that’s all down to WordPress (so, if you can get it working with WordPress…).
2. By default, any user can see any user’s activity. You’ll either want to build some sort of bespoke privacy control, or see if you can get s2member (plugin) configured to work like this.
3. Anything’s possible. BuddyPress 1.6 will have built-in integration for the Activity component with Akismet, which might help get you started. bbPress 2.x also has built-in integration for Akismet.
4. Yes, it scales. Research into how to scale WordPress up to your expected levels of users.