First things first, find out what company you might be interested in working for and their policies. That will help decide what gun you can carry for the company in addition to whatever restrictions the law places on you as well.
Many security companies require semi-autos be double action only, and have a barrel 4" or longer, no lasers or lights, and no external safety or decocker levers. And for revolvers it's usually 4" and bobbed hammer. This is not a law, but companies, like mine, have found that many people who carry guns for duty are not familiar with how to use them and thus force employees to keep it simple as possible to prevent incidents. I'm a supervisor at an armored car company. Today I watched a guy unload a S&W revolver by holding it barrel to the ceiling and shaking it violently. I had to walk over and show him how to press the extractor rod to remove the bullets. We've had 3 negligent discharges this year, one resulting in an injury. There are many, many boneheads who wind up in security unfortunately. Sigh. The gun you choose might have to comply with company regulations like these.
If you're not sure what company you might be working for, just get a Glock 17, 19, Springfield XD, Ruger or S&W DAO semi auto like the Ruger SR9 or P95DAO, or a revolver like the GP100 or S&W 65, 66, 686 that will comply to company regulations should there be any.