(It isn't clear from your posts which machines are actively used vs. spare, so some of this is speculation that you might need to vary)
Your simplest solution would be to get a copy of Mac OS X Server (the 10-user version will suffice since there's only 3 people in the office).
It includes everything you'll need to setup a web server and a mail server for your domain.
Depending on the DSL Modem you have, you should be able to setup NAT to share the IP addresses, with port forwarding to channel port 80 (http) and 25 (SMTP) connections to the Mac OS X Server.
The Mac OS X Server mail applications will handle both SMTP and POP/IMAP for your users and should be able to handle your growth.
In all, it would cost you $500, plus a day or so to get things configured.
Your simplest solution would be to get a copy of Mac OS X Server (the 10-user version will suffice since there's only 3 people in the office).
It includes everything you'll need to setup a web server and a mail server for your domain.
Depending on the DSL Modem you have, you should be able to setup NAT to share the IP addresses, with port forwarding to channel port 80 (http) and 25 (SMTP) connections to the Mac OS X Server.
The Mac OS X Server mail applications will handle both SMTP and POP/IMAP for your users and should be able to handle your growth.
In all, it would cost you $500, plus a day or so to get things configured.