We've started a similar knowledge base project, and after a lot of searching and testing, we settled on Atlassian Confluence as our Wiki.
It has some excellent plug-ins, so our Visio diagrams can be displayed as web pages. The individual pages can be locked down at a granular level. It has a Sharepoint connector to tie in to our Sharepoint Intranet system.
I've directed my team to post two new articles per week, and the Wiki is getting populated quickly. When a job that only gets run every quarter comes along, we get the steps documented. Our internal processes are on flowcharts so the business folks can see what happens when they put in a request. It's been a very helpful tool, and has not had any down time. We even have embedded Spark messaging links by all the user names, so you can contact an author to ask a question.