If it’s merely a discussion forum, then a basic shared hosting plan would be suitable.
There are additional features still available to you such as a unique IP address, SSL certification etc that come at additional cost.
You wouldn’t need to have any DreamHost advertising on your site if that’s what you meant in your second question.
Unless you are a non-profit organization, which would be free shared hosting, but don’t know about linking to dreamhost from there.
Contact info can be done via: http://dreamhost.com/contact/
There is a form on the lower half of the page, select what the message is regarding and it should be picked up by the relevant people.