Multisite Advice

I am asking this because in all my years building and managing WordPress sites I have never used multisite but want to understand it in the context of a directory before moving ahead.

Essentially we are building a GD powered site that will have hundreds but doubtfully thousands of listings. We plan on eventually installing BuddyPress and offering expanded membership services above and beyond what GD offers as well as possibly a forum.

If I set up the site as multisite, I understand that the forum would logically fit under its own subdomain (www.domain.com/forum) but how does BuddyPress fit under the multisite scheme since it is integrated into the main site?

Content is ready to start going on the site so I need to nail down the infrastructure now as I understand going multisite after the fact is not recommended. I just need to be clear on how to integrate it into our site structure.

Let me know if anybody has experience with GD and multisite and what kind of structure you used.

Thanks!