I started with 3 chickens- it's a perfect number to get acquainted! Why do you prefer a tractor? It seems that since they're portable they wouldn't be be as predator-proof as a secure coop and run. If you're wanting to make good compost, the best way is with deep litter. A tractor will just spread fresh "hot" manure on your ground. With deep litter in a coop or covered run area, you only need to change it out once or twice a year (I do once).
Start with a shallow base of pine shavings, leaves, grass clippings or similar bedding and keep adding more every so often to build up a thick base. This will create a little ecosystem that brings in beneficial microbes and other bugs and yummies for your chickens to snack on. Then once you clean out the litter, move it to a pile to finish composting and you will have the absolute best garden soil ever!
You'll also need to get cold hardy breeds, there are many: Brahmas, Easter Eggers, Jersey Giant, Marans, Orpingtons, Plymouth Rocks, Rhode Island Red, Sussex, Welsummer and Wyandottes (and probably more). You could buy a prefab coop for pretty cheap ($300) or less for a used one, but I would worry about parasites or diseases coming with it. A prefab also won't hold up very long, but can be a good starting point to see if you even like having chickens, and figure out your actual needs through experience. Building a coop yourself is usually best for long term housing... might cost anywhere from $400-800 or a lot more depending on how big or what materials you use. You could take a look at what I built... I included a materials list with prices at the end of my article:
https://www.backyardchickens.com/articles/the-mod-coop.74256/
Here are some other articles that may help you get started and think about preparing for winters:
Advice For Chicken Owners
Prevent Frostbite During Winter
Chicken Coop Ventilation - Go Out There And Cut More Holes In Your Coop!
To Insulate or Not To Insulate
I know that's a lot of reading, but it might help you get some ideas what to expect.
Good luck with your research!