Small Business Server are limited to 75 users and single domain, single forest setup. They cannot trust any other domains either. You can have more than one domain controller, so for instance if you have a head office in London, UK and a branch office in the US; then it is possible to host a branch domain controller in the US and have Active Directory Replicated accross.
So to answer your question, yes a 20 user network site with file shares, email and ftp access is completely possible. You can also allow remote access via SBS VPN setup using Routing and Remote Access and in addition, you can setup OWA for Outlook Web Access as well as Outlook Anywhere.
Finally you can share your data and files over the Internet using Sharepoint Services or WebDav.
If you require anymore information, please don’t hesitate to ask.