For a small office ( <20 users), would you consider a hosted Zimbra solution?
You'd get all the benefits of the network version (connectors, the new iSync native sync for Mac, backed-up centrally-managed server that you never have to mess with, and you can (with the right hosting provider) be a domain admin for your domain, giving you control over your users, etc.
All without having to buy a server, or stress out because you (a) clearly like and want the Network edition, but (b) your office is pretty small, and can't afford the $1450.
Hosting plans are about $7/user per month (from the ones I've seen posted). Check around - maybe one less thing for you to have to deal with internally. Just my 2 cents !!