Get a cheap game and learn about them and tinker/fix before attempting a homebrew I’d say. But if u wanted to just have flippers and pops you could direct wire voltage thru the switches to power them, same with pops so no need for a computer or boards if you didn’t need score/logic and other things (controller lighting,etc). Not sure how much enjoyment such a ‘pin’ would bring long term though and even with a power supply, plywood, targets, and flipper mechs (and pops?) you’re pushing up against that 250 mark pretty easily.

If you want to take that a lot further, then yeah as jabdoa said — $$ adds up super quick