Which one do I use if i am to have functions declared in C++?
I need some kind of designer to build the front end view…
Slate, HUD or UMG?
You’d use all of them.
The HUD is just the players view of interface controls and graphics.
The Slate Widgets are the interface screens which you’ve designed using UMG.
The HUD can display the widgets, you just have to tell it which ones to show and when.
It’s pretty straight forward to call C++ functions from blueprints. Do you have anything more specific on what you’re trying to do?
Thank you, no no… That’s wonderful, I just wanted clarification on that. Thank you! 
In general you will use UMG.
The HUD was used before UMG came out. It uses the Canvas to draw things at given coordinates.
Slate ist the Code Version of UMG. The Widgets you create are based you Slate. The same like the BP Nodes are based on C++.
You can expose C++ Function to blueprints and use them in the Widget and i guess you can create your own Widget Classes
that are childs of the UUserWidget or something like that.
I’m having a play with this all right now, I’ll come running if the tutorials aren’t quite cutting it. Thanks for your help dudes/dudettes!