I've setup exchange before but never in a cluster across multiple AD Sites which is what we are now in the planning stages of doing.   So I have a couple of questions.

1) Is a load balancer required in order to ensure high availability of emails being received externally? 

2) Are there any special requirements needed because the cluster will be across AD sites?

The big concern from the partnership is that we are able to continue to send/receive emails even is one of our AD sites experiences an outage such as a power outage.

CAS servers don't belong to a cluster, and there is no notion of a CAS array in Exchange 2013.  While DNS round robin can work, it's not a true high availability option so you would need a load balancer that supports hosts separated geographically if you want high availability.

There are other techniques you can use for different geographies that don't require geo-load balancers depending on what it is you're trying to achieve.

Thank you for that answer. That helps a lot. Can servers with the mailbox role installed be in a cluster across AD sites? My understanding is that they can.

Yes, you can stretch a DAG across sites.