How many users if its less then 50 then you can buy a sSBS server.
If its more then 50 than I would recomemnd you to use the normal server.That is standard windows 2003 and exchange 2003.depening on user count you might need to use enterprise edition as well.
You need to contact your ISP and register a domain asn ask them to create a MX record pointing to your external IP(Firewall or Router)
Then install windows 2003 promote to a dc install a windows 2003 memeber server and install exchange server.
Join all the computers to the domain and configure outlook.