The builder is figuring about 2 sq ft per bird which might work for some people but 4 sq ft is generally the recommended minimum on here (especially in areas with cold/rainy climates where the birds may not want to come out of the coop at times).
For the 7x10, 4 windows + vents should be good as long as they aren't tiny, though if your husband is able to add more later (if needed) that's not a huge concern.
As far as your slope, depending on how sloped it is you can level the base using posts on concrete pads and beams to support the floor, or you can cut into the slope and use a retaining wall to create a stable flat area, though the coop builder probably won't do that for you, so you'll have to see about doing that yourself or hiring someone to help install the coop.
Yes it is a little complicated trying to decide on a coop (I redid my layout several times and then after all that, the company said they couldn't cost effectively do it as I had it, so I had to redo it again) but you'll be happier with the end result when you're putting this much work into it!