What restaurant POS hardware do I actually need to buy before I switch the counter on?

Less than you might think. The app runs on any Android tablet or phone, so a mid range tablet is enough for a POS that sells. For paper you need a thermal receipt printer on your wifi, on your network, or over Bluetooth, because USB printers are not supported. The cash drawer plugs into the receipt printer, and the POS sends the pulse that opens it. A POS machine with a built in printer and a drawer socket works out of the box, but it is not required. There is no charge per device, so a second tablet for the kitchen wall costs only the tablet. The POS and the counter tab in the docs lists what works.