Here's a quick tip that will hopefully save you some time:
As you may have noticed, WSUS can now be installed as a role service in Windows 2012. I'm using it successfully now with an "external" SQL server. If you plan on doing the same, note that when you install the role it will inform you that the Windows Internal Database feature is a prerequisite even though you don't intend on using it.
As you're going through install, feel free to un-check the Windows Internal Database feature. You'll get a chance to specify the SQL target during the configuration phase of the install and all should work right off the bat.