BuddyPress.org

Based on that then yes buddyPress should be an ideal fit, it will fill all your requirements either out of the box or with the addition of a plugin here or there.

As far as I can recall WP was capable of running on different DB engines but I’d have to verify that and may be going mad, best to check the WP codex for info.

I would draw issue on whatever poll it was that referred to WordPress as a CMS it’s a blog which extends beyond that boundary and keeps extending and growing and does do a good job of accomodating static pages but apps like Drupal are more geared towards being termed a CMS imho.