2. The vermin that are gonna live underneath, and the bacteria in the crevices. What you can't see.
If you build a raised wood floor, IMO it should be at least 18" off the ground, higher if you will let chickens have access under there. THat's high enough to provide reasonable air circulation so rot isn't a problem for your floor, and to make vermin feel exposed enough that they won't be hanging out there.
I don't have any real money for this project; however I do some side work and am willing to work extra hours to pay for the bags of concrete and mix them one by one in a wheelbarrow, if it will save heartache later and help keep the chickens warm. Other folks who keep chickens in our area have had them practically stop laying in the real cold weather, when a souffle tastes better than at any other time of year.
A slab floor is of course good too
although unless this is a *small* coop and you are a very vigorous individual, pouring a slab floor by hand-mixing things in a wheelbarrow is not necessarily so practical.
Whether a slab floor keeps chickens warm is actually a very complex subject, but suffice to say that it doesn't always. First, you need a buncha bedding on it or you WILL have cold chickens; and second, in the latter part of the winter when outside temps are warming up sometimes, a slab can make the coop stay cooler than it would otherwise. It is complicated.
A slab *is* good 'n' predatorproof, though!
BTW while extreme cold temperatures probably do have an effect on laying, a) there are other features you can build into your coop to minimize the extreme coldness you get, and b) providing at least 14 hrs of light per day, choosing a good winter-laying breed, and adequate protein to get them through any molt that may be going on, will make a bigger contribution to your winter egg supply overall.
Good luck, have fun,
Pat