I am currently running my AD at a server 2008 Functional level. What do I need to look out for if I setup  server 2012 R2 to run exchange 2013?

Nothing in particular.

Hi sapper12,

Thank you for your question.

We could deploy Exchange 2013 without any problems. If there are any issues when you deploy Exchange 2013, welcome to our forum and we will provide a better solution for you case.

we could refer to the following link to deploy Exchange 2013:

https://technet.microsoft.com/en-us/library/aa998636%28v=exchg.150%29.aspx

If there are any questions regarding this issue, please be free to let me know. 

Best Regard,

Jim

I have finally been able to get exchange 2013 to install successfully. However, When I open the Exchange Admin Center and double click on of the users I receive the error message "Database is Mandatory on Usermailbox". How can I correct this?

Hi sapper12,

In order to avoid confusion and keep track of troubleshooting steps, we usually troubleshoot one issue per thread in order to find a resolution efficiently. Concerning your other question, I suggest we create a new post via:

https://social.technet.microsoft.com/Forums/en-US/home?forum=exchangesvradmin&filter=alltypes&sort=lastpostdesc

If there are any questions regarding this issue, please be free to let me know. 

Best Regard,

Jim