Steps to upgrade 2003 SBS with Exchange to 2012r2 Server with Exchange

I am upgrading my Small Business Server 2003 running Exchange 2005 to Server 2012r2 and Exchange 2013.  I am hoping that I can get a majority of the new server setup while keeping the existing SBS running.  I only have about 50 exchange accounts so I am taking this opportunity to do some "Clean Up" and manually setup all the account since there is not a direct upgrade path from 2005 to 2013.  I plan on backing everyone's mailbox to a PST and importing into their new account.  First question; Can I run them side by side while setting up since the SBS is a Primary Domain Controller?  What are the basic steps or obstacles?