I am trying to figure out the proper failover for Zimbra servers in environment where I have 2 servers in 2 different geographical locations.

My initial idea would be to do mysql master-master replication + ldap master-master replication + drbd on /opt/zimbra/store/ and /opt/zimbra/index/
the dns pointing to zimbra.mydomain.com would have low ttl, in case of problem with primary location I would just switch to the other.
Is this enough for replication or am I missing something?

Second question is about cost (I am using trial version now and plan to go for NE), will I have to pay for 2 zimbra installation or 1 is enough (I would have only 1 zimbra installation running at the same time).