Network environment for Build cross-site DAG

My customer wants to do the Computer room relocation，and needs to migrate the exchange server. Since the mail service cannot be interrupted, he wants to finish by constructing a cross-site DAG. The mailbox data has 5T, but there are two problems. The first is How to  Copying such large mailbox data to the replica site, the second is how to evaluate the network environment between the two places. Can I also use a VPN?
Thank you Very much！