Pin wiki for building pinball is a great starting point. Consider, if you are hoping to do a modern style game complete with art and lots of mechs and animations.. you’re looking at probably $5k invested and years of work. Not sure what your current pin experience is, or what tools you have and coding, anaimstion, etc. knowledge.. it’s doable but a lot of people underestimate the commitment.

I believe the p3-roc is what you want for a custom game from scratch, and then you’ll need the rest of the boards and a power supply, etc.. you can start designing w/out this, get a piece of wood cut and start with cardboard and hot glue. It works surprisingly well. Or start CADing or start designing in visual pinball — there’s multiple ways to tackle this depending on your preference and skills.