First off, I'd recommend that your employer put up security cameras and an alarm system, or revise the coverage that the existing one already has. Hell, even "fake" security cams will probably scare the curs off...
Past that, how big are your hands? Does recoil bother you much? Are you going to mainly be stationary or do you wander?
Scatttergun is going to be tops, but if you are insistent on a sidearm, then Sig it is (IMHO). If you have smaller hands, then a p232 will probably fit like a glove; you may not like the fact that it's a .380, though. In my mind, that's what hydra-shoks are for...
p226 and p239 (if you're into a smaller, large-caliber gun) are all rock-solid firearms.
You may be hard-pressed to find any of these new under $500, though. If you can spare another $100, then you should be in the $$$ range.
No matter what you decide to choose, GET TO A RANGE and shoot a few calibers and a few different types of pistols before you purchase. I can ooze praise for Sig and a few other firearms and calibers for hours on end, but you are the one that has to be comfortable with what you are carrying.
