BuddyPress.org

The transition from WordPress to WPMU is really simple. I did it in a few hours and there’s tons of resources out there on how to do it. Let me know if you want me to link some up and I can definitely do that.

As for your question, there’s no plugins I know for regular WordPress to add BuddyPress-like functionality, but Ning.com and KickApps.com may be worth looking into.