Don't reinvent the wheel here, either. If you are going DIY completely, use one of the existing projects, like the Open Pinball Project hardware. Or roll your own with a microcontroller(I did it this way, because of the constraints of my situation). Use a Raspberry Pi 3 with Mission Pinball Framework for the rules and the overall control. You can do most of the game with just configuration files and no custom programming.
I would start simple. Go to http://pinballmakers.com/wiki/index.php/Main_Page, hit up the "Files Section" and grab a playfield DXF. Cut it out with your CNC machine, mount some aftermarket or ebay parts and work on a proof of concept. I've started with populated playfields, but my goal is also a complete DIY game at the end.

EDIT: LOL, someone beat me to the submit