I'm just going by my own experience, but I have no problem agreeing to disagree.
Whenever I post answers here, I assume the OP to be a person on a budget unless otherwise stated, so I always shoot out the most cost-effective advice I can. With lasers, it is possible to spend a nearly unlimited amount of cash but very few of us have it like that. A multi-watt green system is guaranteed to set the OP back a bit of money.. I'm only giving advice based on the fact that I have tried it before with 200mW and had no problem at all seeing the projection from more than 500ft away. Next time I do a building projection, I'll do it with my 200mW and film the results. It's not blinding, but it is quite easily visible.
If you can't see the beam from a 1W green outside with no fog or smoke, then it ain't a 1W green. I just received a 1W green module and it's like a freaking lightsaber, and that's in downtown St. Louis with TONS of light pollution. Of course, this is a motionless beam. In a scanner it will be less visible of course, but hardly invisible. I have a video of a 200mW green system (along with a 500mW RGB) being used outside with a small amount of mist in the air. You can see that the beam from the 200mW green scanner is EASILY visible with very little mist. This video was shot from ~175 feet. (ignore the crappy music)
After shooting this video, I felt like the standard conception that a TON of power is needed for outdoor beam shows might be a little excessive when you're not talking about professional shows. For amateur needs, I've been able to do OK with much less. The video proves that.
You're definitely right about the building projection in terms of getting the absolute best effect, but I'm firmly of the opinion that it could be done with less than a $2000+ expense.