Then I think you should really either consider Pligg or Joomla with Community Builder. I've had a LOT of experience with the Joomla/CB combo, and that, along with carefully chosen extensions, should allow you to build a complete social networking solution for Dreamhost that will cost you nothing (unless of course you opt for the occasional paid extension, many of which are dead cheap. Actually, the BEST commercial extensions are cheap). Ultimately, given your very specific requirements, I'd recommend the Joomla solution, which you can customize this to your heart's content. Just to be clear, I am not a "fanboy" of any particular solution; I've just recommended what I think is best given your stated parameters.
There are only just a few good social networking scripts around. Actually 3 well known and solid, they exist for many years already and have good support too and private forums. phpFox - http://www.phpfox.com (the best of the 3 if you get the COMMUNITY version with all features, including Marketplace, which is classified what you're looking for.) Social Engine - http://www.socialengine.net Jomsocial - http://www.jomsocial.com (this is a Joomla component, Joomla itself is free)