My first suggestion is not to worry too much about the cold. There are flyers in northern Canada that have very open lofts and do just fine. The birds will handle the cold as long as they are healthy.
Ventilation is something I spent many many hours thinking about. I ended up using grated vents in the floor along the front of my loft. they are one foot by four feet and I have eight of them (my loft is much bigger than what you have planned, you wouldn't need that many). That is for incoming air. To vent the stale warm air I have the space between the rafters in the roof screened off all the way around. I have noticed no stale smell in the loft regardless of temperature. Here in Georgia it can get into the triple digits in the summer and down well below freezing in the winter. I am very happy with this aspect of my design.
As for the floors, I used Luuan over plywood. It is fairly cheap ($9 per sheet), is water resistent and very easy to scrape. I would never use anything else.
Hope this helps some.
Dan