A small store works with one workstation, which can be any kind of Mac you like.
Use the networking capabilities to manage your whole store with Checkout.
Congratulations, you probably have the biggest store on the block.
Connect up to 5 Macs to your wired or wireless network. You can set your 5 workstations up however you'd like. The configuration above lets you run two workstations on the floor, two in the office, and one in the stock room - but this is just one possibility - it's your store.