The hard part I think about the whole deal with a laser gun is it's area of effect. A well focused laser has an incredibly small point even at a good distance, you have to soft focus the beam without turning it into a flashlight, which requires some knowledge of optics, and some way to test it.
Any pre-made modules you buy are likely going to be focused already so it's a batter of putting two lenses in front of it, one to widen the beam and then another in front of it to focus it back into a beam just larger. You could practice with a red laser pointer for visability with a smoke generator or something so you could see the beam spread but you'll have to refocus with an IR laser to some degree. I'm going to guess an IR laser is going to be visable in smoke through a digital cameras viewfinder just like a red one is visably, but you may need to be in a relativly dark room to see it. I'm going to guess you want a spread of about 1-2 feet at your max range, and then you just use a debounce circuit on your detector so that 'flashes' don't count as hits.