How to create a client zone?

I run an online marketing agency and each client has a lot of documentation and files which I would like to provide access to securely through a client zone on the Agnew Solutions website.
I would like to know what plugins would be required to set this up, each client would have an individual username and password which would send them to a page with links to all project documents – however, each file would need to be only visible to that individual account and perhaps password-protected also for additional security.
I do not have a significant budget and preferably the plugins would be free, if you are aware of a solution to my problem please let me know – perhaps a link to a how-to video to assist in setup would also be useful?