I use recipe kits to brew five gallon batches of homebrew and would like to purchase a kit and keep it around during the winter to have when we are snowbound. My question is how long will they be good and what about the yeast in the kits?
Yes, all the ingredients in the kit will keep well, although you should take them all out and store them specifically so that they last the longest. Kits generally use dry yeast which can keep for very long (6+ months) at room temperature and longer if you put them in the fridge. If the kit has hop pellets in it or whole hops, make sure they are in an airtight bag. Then just throw them in the freezer and they will store as well.
However, might I recommend brewing without the kit entirely? All you need is some malt extract, hops, and yeast, perhaps a little specialty malt as well if you want it. Liquid extract can be stored in the fridge. Dry extract and uncrushed specialty malts can be stored in a cool, dry place in airtight containers, but not in the fridge or freezer (you don't want any condensation on them). Dry or liquid yeast will stay viable for 6+ months in the fridge, and dry yeast can also be stored in the freezer. Hops should be in vacuum sealed, airtight bags, and can also go into the freezer. Assuming it is all stored correctly this will all keep for 6+ months, however I definitely recommend making a starter for the yeast a day or two before just to check that it is viable and to get good cell counts. Good luck!
You can store yeast for 6 months or more at fridge temps. But you need to invest time and resources into using a starter if you are going to store yeast for much more than a month post its manufacture date to ensure a good start. (This is not to say that a starter shouldn't be used every time regardless of manufacture date)