Thanks for the answers!
My RetroPie-Project goes ahead and I think I need some base support.
All hardware is available and now the tinkering and programming can start.
So thats the plan:
I have an old rotary phone and want to use it as the case for my raspberry pi 2. Instead of the normal RetroPie Gui I will show my own user interface where all available games are displayed with a “phonenumber”. The user can then dial a number to start a specific game. Probably there will be no more than 10 games.
I think the answers above will help me to start the game, but a few questions are still open:
How can I start my own programm while system start, instead of the RetroPie GUI?
Which programming language should I use? (I have some experience with C#, Java, C and Python)
I’m thankful for every tip.