Personaly i like the to have my webmail on port 80.
, but on some locations where you want to access wour webmail, port 81 is blocked. Port 80 is always available as it is the default port for the http protocol.
You also can host your webmail in the webspace of your choice like http://webmail.yourhostingcompany.tld
which means that you can easily download your site via FTP for maintenance and/or cusomizing it on your local desktop.
Actually, your todo list is very short as the users which you create within ISPConfig for your website will have:
-access to their mailbox via an e-mailclient like MS Outlook AND Webmail as well!
The user marked as administrator uploads the website to URL http://www.yourwebsite.com
and other users to their own webspace at URL http://www.yourwebsite.com/~username
For FTP-access you can use the hostname or any domainname at your server and you must know that you have to upload the files in the /web folder
URL for FTP: yourwebsite
username: name (as defined in ISPConfig)
password: password (as defined in ISPConfig)
I think you can do it with this info!