Seeking Advice on Creating a Community

Hi Folks, I'm creating a test site to bid for my largest web design project ever and this test site is to prove to myself and to others that I can build a community. I've never built one before so this is going to be a very VERY BIG learning curve.

They want a blog, forums, vendor directory, events, competitions and members galore.

I was thinking of creating a wordpress multisite so that one site can be for forums, one for vendors and the main site for community and blogs.

So what is the best thing to do .. activate buddypress across the entire network of site or just on individual sub websites?

What WPMU plugins are you using to power your community driven projects?

Thank you.
Cathie.