I'll try and talk to DH on busgoons when I have the time, he's the big cheese admin on there - I know the Joomla forum software is free but he pays for the hosting, don't think it's overly expensive though.
Thing is though that every forum goes down every now and then - I wouldn't worry about it unless it becomes very frequent.
Setting up a more "proper" forum won't be difficult, but won't be free either.