First off, I'm not 100% up to speed with RE terminology (got started on this board to help my boyfriend, who is).
That being said - I'd focus on getting one thing first - maybe all sellers, maybe all buyers - and then inviting the other group to come through.
There are tools you can do to speed up the process, like Lead Evolution (not mine, I just adore it like crazy), or you can hire people to do Google searches for you and then contact people.
If you do a forum, I'd suggest pre-populating it so it doesn't look devalued when people come in. I'm personally not a huge forum person but when I DO use them, I like to hire the people at WiredFlame (again, not my service, but inexpensive and great work). Their service helps get some introductory posts on the forum so it doesn't look like your site is a ghost town.
As for building it, I personally love Wordpress. It can do pretty much anything, is easy to use, and there's a plugin for everything.
Joomla's another great one but the learning curve is insane!
Hope this helps - please clarify with any specific questions and I can try to target in more. :)