Oh how I could spend someone else's money so fast!!
I have equiterr flooring in my stalls. It worked great for a few years and then it became saturated. I wish we had done overkill on the drainage pits rather than what the manufacturer recommended. I put rubber mats over the top of them last January because we had a terrible flood in 2011 raising the water table so that the stall flooring just will not work properly. I do love it though because it is completely level and the little bit of urine that does leak through the mat lines will be absorbed. If I had that to do over again, I might go with the equiterr, but I would definitely get a continuous Mayo Mat. They are amazing.
I second or third cement floor everywhere but the stalls. Anything else will be dusty. We had crushed asphalt in our storage area and ripped it out to put in cement as soon as we could afford it. You will at a minimum want to consider little cement pads for your stall edges to sit on. This will keep the stalls level and prevent them from getting rust.
We did spray foam insulation for the barn after we had to remove all of the fiberglass batting from the flood. It is freaking amazing; seals up all of the little drafts. I no longer have frost in the corners of my end stalls.
In the horse area, I have white corrugated sheeting for the ceiling. It really brightens the place up. It is just attached to the rafters the same way it would be attached to a wall. My tack room interior and exterior has the same white sheeting. That way you can vacuum or power wash the walls - whatever you want to do. The interior used to be finished sheet rock with white paint. The exterior was carwash board because it was adjacent to the wash rack. I would not recommend the carwash board. It will turn yellow no matter what you do and dust sticks to its bumpy surface.
I would not do a stall door directly to a paddock. Every barn & facility that I have seen where animals have direct access to touching the building has trashed edges of the building. They will rub on it and bend it, some will chew on it, some will get bullied into it. Their poop and hooves will break down the bottom edge and it will look like a mess in just a couple years. Plus if you do this, the horse will choose to poop in the stall when he could poop outside and they won't go inside during inclement weather unless you make them. That is just murphy's law of horses.
A 10ft side wall is incredibly short. I don't know if you would be able to park most trailers in that. I have 12ft side wall in the barn and 14ft in the cold storage area. I wish the barn portion had 14. We elected to put the fiberglass panels along the top edge of the sidewall rather than the roof. It lets in a good amount of light and I don't have to worry about snow, hail, branches etc damaging them on the roof because the soffit protects them.
Are you planning to put in a rat wall? Even if you do dirt floor, you will probably want a rat wall.
Are you going to put a base board to keep your bottom edge of the sheeting from contacting the ground?
If you select the right trusses, you can put a roll up door right to the edge of the barn. This makes parking items much easier than if you have a 6ft space from the wall to the door.
When placing doors be very mindful of wind & drifts. My horse area has an east and west door, while the storage has 2 south doors. In the winter, my west door is difficult to use because the NW prevailing wind likes to make drifts.
Are you planning on storage areas for hay, grain, and equipment like wheel barrows in a way that keeps them away from the horses and out of your way?
How are you planning your water & electrical?
If you are pulling electrical from a main panel or from your house, you should really consider having internet (and maybe telephone depending on your cell service) trenched in at the same time. I LOVE having wifi in my barn.
Also, you may want to check with some local contractors, you can probably have a pole barn built cheaper by getting the material from a lumberyard, Menards, Home Depot, etc than you can with a name brand building like Morton or the others.
I know you say they won't be inside much, I think everyone says that & then their horses end up inside at night pretty much every night, so you might as well make the barn your "home" at home. I know I spend hours every day at my barn.