Trying to add a GUI back to server core (in a lab environment) highlighted that some fundamental issues with update are still broken.
Basically a Core installation won’t copy the installation files locally like a GUI install (to save space). All well and good as you can get the source files from the installation media. Well, unless you have applied any updates, in which case, you are in for a world of pain as the media will be out of date.
The article below explains the issue and how to get around it, but basically, I would just install the full-fat version of Server and then remove the GUI.
A shame, and hopefully something that they fix in Windows Server 2016.