When we delete a user on google apps it will offer to transfer drive ownership to another users. We have File Sharing user that will be the new owner of all documents of the deleted user.

For emails you can export data from Google (https://support.google.com/accounts/answer/3024190?hl=en) but I have no idea how to import.

You might also want to look at Spanning (http://spanning.com/products/google-apps-backup/)