Hello, which device is that you used to turn the camera to look left and right? Looks like some kind of joystick but it seems very convenient for camera movement, well done
Look for parts and datasheets to connect them to an Arduino (can be an Uno or a Mega, Mega has more outputs for gauges and lights) and look for the Gazz plugin that makes Omsi able to output OMSI-information to the real world. Not completely sure about the steering wheel, but it could be a modified G29 or similiar with a different steering wheel attached. I am currently building one for a type of bus that used to drive around here (B10M-55, Duvedec, from Volvo). Be prepared to invest lots of time (and depending on what dashboard you want to build maybe also lot's money) in building the dahsboard, because it will have some parts that will need to be operated through some kind of electronic component, because the equipment that listens to OMSI does not power them from the board directly.
keypad already wired with matrix circuit. so it has 8 pins(4 for column, 4 for row). if you use BU0836 for key input, there are column and row pins. just connect them. i have no idea with other interface board.