Reply To: Not sure what to use
A subdomain is a good idea to ease a lot of extra configurations to keep your forum private from the other site members.
When opening the forums use attributes (private, hidden) and give your members the matching privileges so that a stranger to the site that happens to visit it don’t see what’s going on.
On your WordPress dashboard (subdomain install) at Options –> Privacy block search engines other wise you will have your forums at google listings and other search engines listings.
If you just want those people debating around subjects bbpress is all you need.
If want them to debating on several subjects, and iteracting between them one to one or one to many, maybe buddypress should be better choice and leave bbpress.
Concerning to have your site locked to outsiders there are plugins for that.
