I think I can picture what you mean. Here's what I've done/am doing--maybe you can apply some of it to your project.
Our old coop looks basically like an outhouse. The floor of it is suspended about 3 feet off the ground and is all hardware cloth. The nesting boxes hung off the back with a hinged lid. On the floor I would pile straw (also a thick layer in the nesting boxes) and as the straw became dirty, I would just put more on top. It took about 1 "flake" of straw each time for the 4x4 floor. The cold kept it from getting nasty--it never smelled.
In the spring, I would pull all the straw out, clean the house, and put fresh straw in. I also would have to pull the heap out from under the house with a rake--as the straw was scratched at and broke down, it falls through the hardware cloth, along with the powder from the food pellets.
We had a water warmer in there and a lightbulb on a timer, but no heater. Sometimes, even with the water heater, the water froze. That was only on the very coldest of nights, though, and didn't happen often. Despite the cold's effect on the water, my birds never suffered frostbite of any kind.
As far as critter prevention, well, the hardware cloth effectively kept them out when their door was closed. The first year, we made a tiny yard by rigging up chicken wire just around the entrance. That became a bit of a farce, however, with deep snow drifting, etc. It was hard for people to get in there, too. So after that, I would just open their door for a few hours a day during a time I was going to be home to keep an eye on things. We have all the same predators I'm sure you have.
This summer I built an 8x8 hoop coop so we retired the old "out house". It's situated right on the grass out in pasture and I'm going to do deep litter again, but this time with kiln-dried pine shavings. Since there won't be air circulating through it from the bottom, I want to start with something that's completely carbon; straw tends to act like a nitrogen when it's moist, I've read. Chickens' poops are a nitrogen, so when combined with dried shavings, they compost nicely. When combined with straw, it tends to fester (I love that word. hehe). I do, however, know a dairy farmer who does deep litter in a giant hoop shed that he puts his cattle into for the winter. I don't know how big his herd is, but it's pretty big (he owns Butterworks Farm--do you have their cream and yogurt there?). He says in the spring he has the most beautiful compost. Oh, also, as deep litter decomposes it makes a bit of heat.
Again, a water warmer, a light on a timer, and that's it (this year I'm using a CFL bulb).
Today I begin building an A-Frame coop for my boys (they just really make life miserable for my poor girls!). The bottom half will be hardware cloth all around but I'll staple 4ml plastic up on 3 sides for the winter. The top half will be solid. The plans call for a solid floor in the top half but I'm going to make one half solid, and the other hardware cloth. I just really think air circulation is the end-all, be-all in a chicken house. I'll put straw down on the whole floor in the upstairs. The A-frame will be situated on top of one of my raised garden beds and I think I may put pine shavings down on top of the dirt so it and the poops can be worked in and compost over the winter. Here's the coop I'm making:
http://catawbacoops.com/
Keeping critters out of the large hoop coop... well, there are bales of straw all around and a secure door. I guess I could just keep it closed most of the time and only open it when I'll be around. The A-Frame will sit down inside the perimeter of my raised bed, which has hardware cloth down under the dirt to keep moles and other diggers out.
Whew! Sorry about the novel. I hope some of what I've done can be applied to your situation. It sounds like a fun project!
