I am not an expert on this, but I would suggest not mounting a light on the pistol. Use one of the common grips (see the URL below) to hold the light in your off hand. This allows you to search around with the light without simultaneously pointing the gun at whatever you illuminate (e.g., children's beds to check if they are all right), while at the same time quickly bringing the light up to support your weapon. There may also be situations where you do not want to give the position of your gun away with the light. For many years it was taught to hold the light away from the body, as that is most likely to be what draws fire from an opponent. I don't know why the change was made - possibly to give better control over the handgun with a brace from the weak hand.
This also means that you do not have to have a special holster for the gun/light combination, and you can carry the light in the holster or holder that comes with it next to your spare mag on your off side.
If you attach the light to the weapon, be sure to test fire it. Some will not hold up to the recoil or the slide returning to battery. I have read of the end caps coming off. I had a compact laser that turned itself on, whenever i fired my semi auto.
Surefire and Streamlight seem to be the lights of choice.
Here is an excellent write-up (IMHO), which recommends using the "eye index technique" to hold the light in the off hand and discusses many other points -
http://artofmanliness.com/2012/11/07/how-to-use-a-tactical-flashlight/.