There is a blog function built into ClassiPress, but your ClassiPress installation will by default, look like the
ClassiPress demo in terms of its appearance as a Classifieds website (not primarily a blog).
WordPress is the platform on which ClassiPress runs.. so yes, all the admin/back end of the site is WordPress.. you can see how this looks/works from the
ClassiPress admin demo.
What you wish to achieve is very straightforward, and if you're not concerned about linking user accounts from the forum to the ClassiPress installation, it is even easier. Effectively you're installing WordPress/ClassiPress in a subdirectory:
www.yourdomain.com <=== Your forum (presumably your main homepage)
www.yourdomain.com/classifieds <=== WordPress installation with ClassiPress
Create the links between the two, and that's it.