If you have a client/server setup then you can use the server as a firewall but it is easier to use the switche/s and a router with ACLs setup instead of a firewall.
Server 2003 SBS is not cheap so it may be wise to look into Linux? Linux is much better than MS server 2003.
Connect all clients and server/s to a good switch e.g. Cisco 2950 and then to a Cisco SOHO router
Yes use DHCP, either by the server or even better by the router but it is simplier use and easier to troubleshoot.
Remember just because you can do something doesn't mean you should, best keep it simple, esp if you are admin!