Well, you'll need
HTML and XHTML to make the core of the site... These are your foundation.
XML if you need to syndicate.
SQL for the database where you'll keep all the data.
PHP to manipulate the database.
PHP is pretty much the standard this days, so it's always a good choice, but don't forget the (X)HTML because it really is the foundation.
Keep it simple and functional. The one requirement in the market is a simple, functional platform that anyone can use.
Keep me posted on your projects. Maybe we can work together in the future, I have a few projects of my own on the back burner.