Hiring a security Guard will be extremely costly to your shop. I highly suggest tho getting a security system, a good one too that can secure your building and records everything. That will be more useful then hiring an expensive security card. If protection is something you are worried, maybe bring pepper spray or a taser to use ?
Or better yet, find a location for your coffee shop that is not in a high crime zone where you need to wear a bullet proof vest just to walk to your car.