You've probably seen by now the issue of getting the right SP1 before you can apply the April 2015 CU.
List of “Good” SharePoint 2013 Service Pack 1 Packages
I use AutoSPInstaller to do my builds. My first attempt at using the April 2015 CU failed with a build dependany failure (as predicted).
In the end I refreshed my install media with the MSDN “SP2013 with SP1″ [which is “broke”] and then extracted the SP1 patch into the \SharePoint\Updates\ folder on my media. Then added the April 2015 CU to the (AutoSPInstaller) \Updates\ folder.
This worked like a charm. Though it does mean my install media is now ~7Gig
Hopefully that helps some people.