You still need to buy a license for Outlook, there isn't one provided with SQL Server 2000. I don't know of any technical advantage to using Outlook 2000 over Outlook 2003... I'd use whichever one is standard in your organization, and I'd use 2003 if your organization uses both.
Microsoft recommends using Outlook 2000 instead 2003 if you are using SMTP or POP3 mail server. The reason for this is that in 2003 the MAPI spooler runs in process - ie. Outlook must be running. Outlook 2000 doesn't have to be running for SQL Mail to work.