I would think with that hardware specification and number of users you'd find it pretty close to real life - it should also be OK with more users than that. The important thing with Zimbra is RAM and you've got plenty of that. If these users are new users with a different domain than your Exchange server then you'd only need to take note of the
Split DNS requirements (including /etc/hosts & resolv.conf), if the users are from your current domain (using the same domain name) then you should also read the
Split Domain article. Other than that, have a look around the forums/wiki for some advice on installing and configuring Zimbra and have fun.
