Starting isp and going to use Debian

["Chris Hoover" <revoohc@sermonaudio.com>]

Me and some friends are looking into starting a local isp.  My friends are
networking experts with some linux experience and I am the linux expert with
some networking experience.

Anyway, my question is what software do most of you use?  Obviously, we have
decided to use Debian for our base os.  However, what do most of you use
for:

1. Webmail
2. Imap/pop access
3. User management
4. Accounting/Finances
5. Drive usage control (i.e. user only get 10M for mail and 15M for web)

Any other advice sould be appreciated.

Chris