A great site for finding the Wiki you want.
I sort of stumbled across this website that gives you the ability to view and compare all the different kinds of Wikis on the internet, both retail and free. I would definitely recommend you visit this site first and choose the wiki you want/need.
It gives you a very good sate of information, from licensing to security, coding tags to server requirements, etc. It really tells you everything you need to know. Also, you can complete a "Choice Wizard" that helps you determine the kind of Wikis that are right for what you want.