You can find some 3rd-party Outlook add-ins at
Slipstick.com
I haven't used any of those tools, so I can't comment on them.
If you DO decide to get an Exchange Server, take a hard look at SBS 2003. It's MUCH easier to use and maintain, has many more features, and costs much less for a small office than does Server 2003 + Exchange 2003.