A couple of years ago, I did some research on Exchange alternatives for sharing email/calendars/contacts within small companies. The system that Outlook 2000 offered apparently wasn't very reliable.
You might find some useful discussion of the alternatives on
Slipstick.com.
We decided that, in the end, we'd be better of with an Exchange Server, so we never actually tested any of the alternatives. Some of them SOUNDED good.
If you only have a few users and have no other need for a Server, you might look at hosted Exchange accounts. They run about $15 per month per user. Most of our users in our own company's SBS server run Outlook remotely, using Outlook RPC over HTTPS. This is, effectively, the same as using a hosted Exchange account. It works fine. Using hosted Exchange, you can also use Outlook Web Access and Public Folders for shared Contacts.
Your client should also consider the advantages of running Windows Small Business Server 2003. That includes Exchange, SharePoint for cooperative workspaces, full remote access and email access from ANYWHERE, strong security by default, and automated backups. Dell is selling SBS 2003, Standard Edition, servers, complete with software, for $600-$800, although I strongly recommend that the initial configuration be done by someone with prior practice installing SBS, to avoid common first-time installation mistakes.