Old post, but as I'm somewhat in to keyboards I'll reply:
Ema2019:
Which model of Arduino is suitable?
Any Arduino with 32U4 chip will do the job.
Ema2019:
exist an example freeware source code?
Yes, in Arduino IDE> Examples>USB>Keyboard/Mouse
There is plenty examples how to do that in Youtube.
Practically any buttons, sliders, potentiometers/rotary encoders will do the job.