I recall reading that Windows 2000 was limited to 10 users as a server. e.g. only 10 users could access it at one time.
I have one client, with 5 users, using a Windows XPP workstation for file sharing and a Windows 2000 workstation for the Print Server.
The XPP workstation is the office admins p.c. She does most of the word processing in the office. She also backs up the shared directory to CDROM EVERY DAY!.
A second client is using a Windows 2000Pro machine to run a Networked accounting/POS system as well as the file/print server. No problem.
Unless you have major security concerns about network access etc. I don't see the need for anything more. You still need to set up user accounts for each usr at the workstation, and yes you can not change the passwords remote like you can with windows server but neither office is concerned with password changes.
I would agree however , if you forsee the user base increasing dramatically in the near future, you may want to go ahead and install Windows Server. You don't need SBS unless you plan to run your own mail server and or Proxy/firewall the internet connection. If all you want is file and print sharing then just install a straight forward Windows 2003 server.
SBS is Windows Servers with Windows Exchange, Windows Sharepoint server for collaboration, IIS Webserver, Outlook Client, Fax Serve and SQL Server.
If you are currently using your ISP for your mail accounts I wouldn't bother with SBS, its overkill.
Whichever way you want to go I would take advantage of having the files centralized to implement a schedule of regular backups.
I would recommend http://www.backup4all.com/. It backs up to CDROM/CDRW/DVD. Not TOOO expensive.
It just compresses everything you have listed to a zip file and writes it to disk.
Restores are easy. Just open the zip file and extract the files you need. Nice and simple for a small office setup.