Q: How much overhead should be factored in for snapshots?
You will want to work with the array vendor you are specifying to ensure you are meeting their best practice guidelines. At a minimum, if they are keeping block-level changes, you'll want to reserve some small percentage of space for snapshots to ensure you can always take them when you need them. Some vendors require setting aside a significant portion of the storage for snapshots and/or replication, so you want to be sure that you've factored that in. Also, beware that some vendors that provide "unified" or all-in-one SAN and NAS solutions have different best-practice guidelines for the NAS data than they have for SAN, so make sure to account for those differences when sizing.