Hello Jim,
If it is only 12 users then I would suggest that you actually use the Watchguard secure VPN capabilities.
Otherwise you would have to move the server to the DMZ or install another server hosting the SharePoint application.
But since you are running it on a SBS server then I would say that it is out of the question in moving that server anywhere since it most likely are hosting other services such as Domain Controller, Active Directory etc
I have used the Watchguard Firebox pretty much and used the VPN possibilites extensively and it has worked perfect and since you can encrypt the tunnels with 3DES that is good in security perspective.
When it comes to "public" computers the VPN might not be the best solution since they would probably have to either setup a VPN client or use the PPTP connection to the Firebox and doing this on a public computer could possibly be a security risk (it all depends on what "public" computer means). As you can see each solution has its Pros and Cons.
I am having users coming in via VPN to some of our SharePoint environments and that has worked pretty good, but usually the users are mobile and are using their own laptop och have a VPN connection setup at their home-pc.
Regards,
Thomas