I usually buy a quart and then freeze whatever I have left in 1 cup containers, as I have found that most recipes call for 1 cup. It will keep, the only change I have noticed is that it appears a little watery after it thaws, but it has not affected the use.
I frequently make whole milk by blending heavy cream and usually skim but I've used 2% as well. For buttermilk, 1 tablespoon white vinegar in the bottom of a measuring cup, fill up to 1 cup line with whole milk, stir let sit a few minutes, stir again before using.
I keep powdered buttermilk on hand for baking. It doesn't work so great for biscuits or pancakes, but works great in cakes. I just purchase whole milk when I know I'll need it for something. I also have the powdered milk on hand, but that doesn't work for things like pastry cream.
I do both! I keep powdered on hand but I also buy the quarts and freeze the leftovers. As mentioned above: for cooking, it works 100% perfectly. There is ZERO difference.
I don't think using 2% instead of whole milk will alter your results. I used to always cook with 2 or even 1% until I started buying whole milk for my DD and thus had it in the house. I even did a pudding recipe - where the milk is the key ingredient of course - with 2% and it was delicious. Maybe, maaaaaaaaaaaaaaaaaaybe a little less rich, but that was for pudding fer crying out loud. For a cake or icing, you will find little problem. I would think maybe that skim would have a potential for a problem due to the lack of fats, but if it works for IndyDebi then I would recommend you take her word for it! 
I've never heard of powdered buttermilk but will have to investigate. Hadn't considered freezing either. Thanks for all the help! BTW, can you substitute sour cream for buttermilk or is that a total no-no?
Powdered buttermilk is found in the same aisle as the regular powdered milk, normally in the baking aisle. I buy a brand that comes in a small round can that is red and white, it is called Saco. Keep it in the fridge after opening.
As for subbing buttermilk for sour cream, it would depend on the recipe. Sour cream has more fat than buttermilk and could affect the texture and end result.
I've never heard of powdered buttermilk but will have to investigate. Hadn't considered freezing either. Thanks for all the help! BTW, can you substitute sour cream for buttermilk or is that a total no-no?
Powdered buttermilk is found in the same aisle as the regular powdered milk, normally in the baking aisle. I buy a brand that comes in a small round can that is red and white, it is called Saco. Keep it in the fridge after opening.
As for subbing buttermilk for sour cream, it would depend on the recipe. Sour cream has more fat than buttermilk and could affect the texture and end result.
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