It sounds like predator proofing will not be a factor with this so that's good. Though I would recommend thinking about protection from aerial predators if they are in your area. So the other factors to consider in a shelter are space, protection from weather, and ventilation. And durability and cost should probably be at least a small factor. Breaking it down...
Space: You're all good there, you'll have more than enough space.
Protection from weather: Ducks are fine in cold weather but they should have a place to get out of the wind. This would protect from wind and rain as long as the weather isn't too severe. Not sure how much snow you get but I doubt this thing would stand-up against heavy snow.
Ventilation: This is going to be a main concern. Good ventilation means good air flow. If you live in a warm area you could just leave the front part open and create an opening in the back. This would allow air to easily pass through. Maybe replace part of the wall with hail cloth or something like that. If you want to leave the door mostly closed and the walls intact, you could use your pvc pipe/fan idea. You might want to look into roof turbine vents. They are designed to draw air up and out. You may be able to figure out a way to use those and then you wouldn't need electricity. Or perhaps a few of those solar powered vents designed for RVs. I image those are pretty light and they aren't too expensive.
Durability & Cost: I can tell you without a doubt one of these garages wouldn't last a single season in my area. It would either collapse under heavy snow or get blow away by high winds. It you have to continuously replace the garage it won't save you money in the long run. But this will depend solely on the weather in your area. If your weather is pretty mild, this garage sounds like a good deal.
If you are still considering flooring I suggest looking into the deep litter method. It works great on bare ground, it's low maintenance, and if done correctly has no smell. I use it in the majority of my run and I love it. I used to just mix it up once a week because the flat duck feet pack it down. Took like 10-20 minutes depending. Now that I have chickens too they do all the work of turning it. All you would need to do in install a lip at the garage door to keep the bedding in.