It seems to me, reading these replies, that powdered milk must vary rather a lot, according to where you live.
I have an unopened packet of powdered milk on my knee as I type. It's Full Cream Powdered Milk, but we can also buy skim powdered milk, and other forms of milk powder. We can get it in several brands, including generic brands.
On the packet, the ingredients are listed as 'full cream milk, emulsifier (soy lecithin), antioxidant (301), vitamins A and B'. It's very reassuring, after reading your horror stories, to know that our milk powder in Australia is just that - milk!
The nutritional information (which I won't copy in full - it's hard for old eyes to read), indicates that per 100ml, it contains 3.8g total fats.
I bought it about a month ago, and it has a Use By date of May, 2006. Instructions are to 'store in a cool, dry place'.
All of which tells me that powdered milk DOES contain fat, and that it DOESN'T keep forever - it CAN go off - in fact the shelf life is probably only about one year.
As for price, it works out at a little over $AUS1 per litre, a little cheaper than fresh milk.
Mixing instructions are 'Add 1 level cup... powder to 3 cups of hot or cold water to make 3/4 litre of milk. For best results, add powder to water, not water to powder. Stir briskly until dissolved. Once mixed, keep refrigerated.'
I rarely buy fresh milk. I never drink it. For visitors who like milk in their tea of coffee, I buy the UHT milk in tiny containers, which most people find more palatable than powdered milk.
I use powdered milk quite a lot for cooking, however. I make things like custard, rice pudding etc, and when mashing potatoes, I leave a little of the cooking water, and add roughly an equivalent amount of milk powder. I'm rather generous with it, because I like my mashed potatoes creamy!
In other recipes, cakes, biscuits (cookies), scones, breads etc, I simply measure out the same amount of powder as water, when the recipe calls for 'milk'. I mix the powder with the dry ingredients, and add the water along with other liquids according the recipe I'm using. That way there's no waste - I'm too lazy to mix up a batch of milk first!
I actually reared my daughter on full cream powdered milk. I was living 'out bush' and it's all that was available at the time. (Going back several decades!) She's always been remarkably healthy.