I don’t think you need to run ADPREP unless you are upgrading to a newer version of Windows Server. I think you should be able to add the second domain controller running DCPROMO without any preparation to the forest or domain. The only special thing to take into consideration when promoting the second DC is that you should select the “Additional Domain Controller for an existing domain ” option when running the wizard.
You could find this guide useful.
As for the Exchange migration, I hope somebody else will help.