There are a few lights/lasers that will fit directly onto your Glock, if it is a 3rd generation (Has the tactical rail). If its not a 3rd gen you still have a few options (laserMax, Crimson). I recently purchased a TLR-2 and I absolutely love it. It is a great training aid and was sited in, in about 10 rounds. As for the brightness of the light, I am amazed at how bright this is. This will be more then enough light to light-up what ever you need. Easy to adjust, and very easy to turn on/off. Some people prefer flashlights, its all in what you like. If you like it and you can afford it, more power to you. I listed a link below that might help you; I would consider the TLR-2 before you bought the M6. I also posted a few pictures of my G22/w TLR-2.
Information about the lights.