I am not sure that type of waterer can be filled without flipping it. Any breach of the upper part will cause any remaining water to flow out the bottom.
The flip side....
If you have a floor close to the ground the vermin have a nice cozy spot to set up camp without being disturbed or potentially eaten by the birds.
I built at 24" on center for my studs. I used pallets for the floor framing and that let me use 1/2" plywood for the decking on the floor. It was a bit of a pain getting the pallets the right size for my needs but it worked.
Look for companies in your area that sell salvaged and slightly...
They can be either or both really. There are a lot of great examples here.
https://www.backyardchickens.com/threadloom/search?query=Hoop coop&tab=620
This one is a great example of it being both coop and run.
https://www.backyardchickens.com/articles/ms-biddys-cattle-panel-hoop-coop.73755/
You can have it steep if you like a steep roof. I prefer less for wind to grab. A 6" drop would be about as shallow as I would want. Go with what feels right for your area though.
As to to big....it really is just a foot longer than your drawing. I have never had a bird complain about to much...
I forget if there is a size constraint or not. I find myself wondering if there is a reason you aren't building a 4x8 structure you can actually walk into.
I would be putting the 4' end board inside the long boards. Cut to 45" of course.
While awesome that you sloped the roof, with that short...
https://www.backyardchickens.com/articles/
Check out the coops in the link above. I am sure you will find a lot of valuable information whether you decide to build or buy.
Numerous members have included step by step pictures and instructions.
How much construction experience do you have?
Tools?
I am pro build.
The prefabs are overpriced and 99% are poorly designed using cheap thin materials.