Exchange 5.5 Implementation…

Hi Guys,
I want to set up a exchange 5.5 server for a small business for 25 users. They want it for their interoffice e-mail as well as their Internet e-mail. I already registered a Domain name for them and I built the server with the same Win 2K domain name as their Internet Domain Name. My question is How do I set up their exchange server for ther Internet Mail connector? Do I need a Static IP address for that ? Any good white papers on it ? or any personal experience. I have knowledge on managing the exchange server BUT I don’t know how to Implement it….
Thanks in Advance !!