I did a little bit of research which microcontroller I should use. Becuase using USB on an microcontroller is new to me it should be also well documented. And to fit this project It  should be relative small.

First I thought about the Atmegegas with USB (32u4, 32u2...), which are still a backup possibility, bur so far i Will concentrate more on Microchip's PIC16F1454 . Resons for that are:

• The "MPLAB Xpress PIC16F18345" development board is using this PIC as USB interface.
  •  I do have one oft this boards.  So I do have already hardware for first Tests
  • On this board the programing interface of the PIC16F1454 is routed out on testpads
  • One button is connected to IO of the PIC16F1454 
  • Two LEDs are connected to IOs of PIC16F1454 
  • This bord although made for evaluting the PIC16F18345, fits perfect my needs for testing, Just the PIC16F18345 needs to ignored
  • Picture below shows this board 

• Available at low price in huge number of different packages (including dip for prototyping or QFN for PCBs) 
• There is an reference design by Microchip about bulding a keypad
• As alternative to Microchips USB-Stack (which looks like not being open) the free and open source USB stack "M-Stack" should run