Install the box, same version Unity you are using now.
Join to new domain
attach to new mail server
DiRT backup old server
Dirt Restore to new server. If you use this route, it will create new accounts, import their settings, back in, etc.
There is a tool for mapping old alias to new alias if you have already created new accounts or have existing on the new system. You read more about this at unitytools.
I wouldnt worry about using the drives as a "tool" to migrate. I would use it more for a backup process.
Search the forums here, a couple of us have written and posted the links for Unity migrations.