What you are proposing to do is actually very easy with Dreamhost, and it can all be done through the Control Panel.
Just follow these steps:
1) In the Control Panel -> Domains -> Manage Domains screen, add a fully hosted domain for the charity, and define a "New User" for the domain.
2) When the domain and user have been created, go to the Control Panel -> Users -> Manage Users screen and selected the "edit" link adjacent to the new user's name you created in step one above.
3) Select whether you want shell, ftp access, etc, which PHP to use, etc, and set the maximum allowed disk storage (which includes all email) for the user and save the form.
4) The last step is to go to the Control Panel -> Status -> Bandwidth usage screen and click the "disabled" link under the "Bandwidth Throttling" column adjacent to the domain name you created in step 1 above (the domain you wish to limit). Complete the following screen, paying special attention to the instructions and warnings, submit that completed screen, and you are done.
Piece of cake! Disk space is controlled by user (which accounts for email) and bandwidth is throttled by domain.
Because this is a separate user from your main user, your files are not available to them.