I work in retail and this is just MY opinion, you want 3 systems and none of them need to be 'servers'.
First, a POS (Point Of Sales) system that you tie your tills\sales registers to. HP, IBM etc. all make specialised POS systems, add registers as required, sales software included, some will export to Simply or QuickBooks, touch screen etc.
Second, a dedicated computer for your camera system with tape drive and\or off site back-up, your cameras (if decent quality) will cost a lot more than the system you'll need, i.e. an i3 or i5 just make sure you get a couple of 1+ TB drives, storage is the issue here, the more cameras and higher definition will need more storage.
Third, your 'Back Room' Admin system with your Office productivity tools, accounting software etc., again, you don't need 'Big' horse power.
Remember, not only the expense of the cameras, but you should have 2 switches, one for the security\camera system, and one for the POS\Admin\Interact connections.