Yo Zimbra team/admins/users/etc !

Recently i tried a zimbra open source edition on a tiny/old desktop computer (p4 2.4ghz 512MB). For sure, it was pretty slow and server startup took about 10minutes. Buti i was impressed how it looks and feels! At the moment i misses only one thing, ability to create domain admin with multiple domains
So, now i decided to migrate our old mail server to the zimbra.
I have something about 2000 user accounts and about 50 lists. Also i want to run mail store over iscsi from nearby zfs san.
What kinda of hardware i need for such amount of users?