It works fine with either internal or external display ( if you want to render things on the notebook's display then make sure you get an NVidia GPU that works with Optimus - not sure if they made it work on Windows 10, I had to revert to 8.1 in order to continue rendering things on my notebook's screen -> Note: using an external display for rendering should yield better performance overall ). I'm using a 13" rMBP with a ViDock that houses a Zotac GTX 760 Amp! Edition and I'm using the notebook's screen for rendering ( no external display ); been working like a charm for almost 2 years now.
@OP: make sure that the setup can provide enough juice ( power ) to the card / it can be hooked up to a proper PSU if needed... I see they have a 75W power brick but that might not be enough ( haven't read up on the card's power consumption but I have a feeling it will need more juice ).
Also, double-check the dimensions, making sure the card you decide to go with will fit nicely into the case.