I work for a large local government organisation who are about to embark on using SharePoint to replace our ageing intranet with an all-singing all-dancing collaborative sit
Many public organizations use SharePoint for publicly-facing internet and collaboration sites. When architected, designed, and implemented to your specifications, SharePoint can meet the requirements you have described (including the accessibility requirements).
The real gotchas with SharePoint (as with any major software engineering project) are that you have a team of competent professionals who know what they are doing.
Here are some links to lists of sites that use SharePoint to get a better idea of how existing organizations are using SharePoint: