Stay away from the Surefire M900, and you should be good to go.
A vertical foregrip is legal. But your firearm has to be registered as an AOW. Not worth it, when the only reason for a pistol build is that you don't want the hassle of creating an SBR. The NFA doesn't cover lights, lasers, tactical vibrators, or pretty much anything else you may wish to attach to your rails.