Where you put your forum depends totally on the structure of your website. Do you need a custom homepage? If yes, it'd be advisable to put your forum in a subdirectory or subdomain. In my opinion, Google's evolved a lot in the recent years and the structure of URLs doesn't matter. I've observed that high quality and fresh content trumps most of the other factors.
But one thing I'd strongly recommend is that once you setup your forum; NEVER change the URL structure. Google absolutely hates it.
There are third party addons which you can use - XenPorta 2 PRO for example. There's one by Brogan called "CTA Featured Threads and Portal". But you will be restricted by XF's post editor to compose your articles.
True. In fact, I've changed our URL structure twice. The problem I've experienced is that you cannot really tell how will Google react. If you setup 301s properly and Google's nice to you - you're all okay. But having been through several ups and downs in the past decade, my advice would be to decide what the site's structure is going to be like and then don't touch the URLs.