a nice loud bell or buzzer that'll sound when the door opens, maybe with a light wired into the circuit as well -- I am sometimes running machines in the back room, and while I try to be aware of the front door, it's hard at times
Given this, I think a buzzer of some sort would be a high priority - unless your dog alerts you consistently, you need to know when someone enters your shop.
It's your choice, but I would definitely have that firearm holstered on my person at all times, if possible. And unless you're an MRI technician, it's most likely possible.
God be with you, in any event.