The G32 in .357 Sig will work fine. What I would put emphasis on is keeping it on your person in a good quality holster. If you can open carry, that will be a faster draw and presentation. When you train, try to train at a range where you can draw from the holster, and also dry fire from the holster at home making sure to have an unloaded gun. Also, I would focus on moving while drawing and moving while shooting. If your shop is hit while you are working they will come in hard and fast, maybe shooting at the same time. A step to the side while drawing may keep their first shots from hitting you giving you time to draw and fire.
Several good ideas in this thread, including the shotgun (I like an 870P loaded with Federal FliteControl 00 Buckshot myself) behind the counter and also having several handguns a few feet apart all around the display cases.
Asking for advice/opinions and thinking about a plan are good first steps to better protecting yourself, your customers, and your staff. Good job.
Just my .02,
LeonCarr