It's a bog ordinary Wordpress multisite installation. The most exotic thing about it was WP-supercache and WPEngine have clever inbuilt logic to safely disable that when you migrate across.
If you're looking to upgrade in the future, you might be much better off throwing a dedicated server at the problem and setting a nightly backup to S3. Also, thank you for posting.
Monthly cost is about $40 ... unless you count my time.