Every cafe near me uses an iPad point of sale. Can I run yours on the iPad I already own?

Yes. GetMenu POS is in the App Store, so the iPad you already own can be a POS that sells or a screen in the kitchen, and an iPhone can be a waiter's phone. Install the app, add the device under Settings → The POS, and type the six character code into it. It is the same app as on Android, with the same menu, staff PINs and reports. One difference: from an iPad the receipt printer must be on your wifi, because Bluetooth and USB printers do not print from an iPad. The cash drawer plugs into that printer and opens from the POS.