May I be the first to suggest an alternative to MediaWiki:
http://wikka.jsnx.com/HomePage
It's called... Wikka Wiki, and it seems to fit just about everything that you want the wiki to do. Security is available et all. Don't know about the editing part of it, though, it seems to use standard Wiki notation, which is all a bunch of garblegook anyways. But it's easy to pick up on, I promise
