Electrics first - or at least test everything well so you know exactly what works and what doesn't, in case it's something you can fix while taking things apart.

When something doesn't work after your shop / restore / .. you at least know it worked before and it's something caused by you and not an issue that was in the game before (a short, something plugged in wrong or forgot to plug in, ..). Or if you didn't touch the boards and it worked before you know it's somewhere in the wiring on the playfield and not in the boards.