BuddyPress.org

Yes, you can just do one blog even though you installed multi site. That’s what I do on http://www.givingsports.com (which I just launched).

The choice of using the original homepage or not is harder. However, you can keep your original home page and still have all the BP features around it. That’s kind of what I did on the site I just linked. I have a unique home page and then the blog is at http://www.givingsports.com/blog even though it’s all installed in the root of the domain.