Once it is all built it does hold more than it appears. The directions are pretty clear, and extra parts were included which was nice. Make sure it is perfectly level at the base or it will not go together as well at the top. Our door and roof vent do not close all the way due to a slightly off base, but it still functions just fine. 
It seems flimsy as you as putting together, but once it is completed it is pretty strong and holds together well in the high winds and spring storms.
If you think you are going to want to put in the additional wall vent. I suggest adding while first installing as it would be easier.