Permissions and security

I am thinking of putting a blog on my website and am concerned not to open some sort of loophole into the site as a whole through inserting a blog into it. Am I being paranoid or is it a possibility? If so how can I protect the site?

Second, is it possible using WP to accept posts ONLY from specified addresses rather than exclude certain addresses?

If it is, is it then possible to set up areas where “invited” people can comment with links to areas where “anyone” can post?

Or is the only secure thing to allow no posts except from me?!? (well, it is me that wants to write and publish my thoughts!)

Thanks