Forum Plugin for Specific Use

Greetings all.

Have been scouring every available WPMU forum I can find looking for a scenario similar to mine - in the absence of a clear answer, I thought I'd start a new conversation here in case anyone else is in the same boat as I. Forgive me if there's a neon sign pointing to exactly this question somewhere else - tried to find it, couldn't.

Running WPMU with Multi-DB, and looking to run a sitewide forum JUST for the main blog. I don't want each individual blog to have its own forum - I just want everyone with a membership site-wide to have access to the MAIN forum. It's easy enough to put a link to the forums in the right sidebar of my theme, but my question is twofold:

1) Recommended forum plugin for this application?
2) How do I make sure that my existing user table (5200+) gets imported to the new forum (without having to log in separately), and that any new users on *other* blogs get added to the forum on *my* blog as well?

My gut tells me that Buddypress and bbpress will be the most common response, but if you forgive my ignorance, I'm not 100% sure how to make it work in THIS application; I was under the impression that the Buddy/bb combo is the answer for giving each blog its own forum.

Thanks!