More to the question, how many people need access to update the system...
A CMS which offers flexible but secure access for users in the charity to add content or moderate public interactions in the built in features like message boards and so on... Depends what you are looking for.
Make a list first and get a sense of the admin structure you want to have in place (in case those elements at some point may prove useful so don't dismiss them)
Then look at a few, as in install and configure and have a look at a few.