Outdoor cat advice

We are thinking of getting a cat (or 2).  Strictly to stay outdoors.  Goal: to control voles and mice.  We have several friends who have an outdoor cat in our climate (cold - Rocky Mountains of Idaho at 6200 ft).  So we've learned about straw bedding in an insulated small box inside a shed; and a system to keep the water from freezing.  My question is food: what is the most sustainable food that we could give the cat? I don't like the idea of buying cheap catfood produced from big ag products.  Are any people food scraps good for them?
Any other advice on an outdoor cat?  How to find a cat that is good at hunting?  Is it a good idea to get two?  (I was thinking they could keep eachother warm).  
We have 2.5 acres and I don't want them pooing in my garden beds.  Should I place a sandbox near the shed?  

Hi Allison;
I suggest two as they will hunt together as well as keep each other warm.
We have two brothers, both fixed, and they live outdoors.
I gave mine a straw-filled brick house and they did not like it, they prefer the large dog bed I placed upstairs in the open barn.
This winter it went down to --25F for a few weeks I allowed them to stay in my shop where it was warmer than the barn.
Mice, pack rats, birds, snakes, I even watched them stalk wild turkeys.  
We feed them  
Purina ONE True Instinct Natural Real Chicken Plus Vitamins & Minerals High Protein Grain-Free Dry Cat Food, 14.4-lb bag
This keeps them healthy and happy.  Unhappy cats will leave. Happy cats hunt...

Giving them a sandbox would help, but cats like gardens and the only thing I find helps is chicken wire.
 

Cats are considered 'true carnivores', and thrive on meat, fish, and poultry - preferably raw, but that's not as big a deal. That said, I've had cats who, though well fed, still loved fruits & veggies, and even one who ripped through the trash, to scrounge dried up, cooked butter beans, lol.

Thomas has the right of it!

Alison,

I have two male cats - brothers. They are outdoor/indoor as they like. They never ever got a taste of any cat food. Since they were kittens, I was feeding them chopped ground squirrels that I hunted, roadkill snakes, gophers. They love to hunt and eat their pray whole. They love chicken bones, fish, milk, cheese.

cats are one of the only pets that will naturally assimilate to the wilds. I know for my pride of cats the dry kibble feeding every day is probably more of a social thing than a nutrition thing. I had as many as 9 but now have 6 and there are some kittens every year and some cats and kittens always disappear probably because of coyotes, hawks, vultures and feral tom cats that come around and want to be the dominant male. they dont need any heat in the winter and will hunt around and find thier own wild food all the time . it's truly survival of the fittest and the ones that learn ti keep as keen eye out and climb high and hide well when predators come around are the ones that survive.
give them some food and they will know who loves them and bring home presents of mice, rats, rabbits, squirrels,  birds and chipmunks to leave on your doorstep.