Baked goods made with whole milk taste richer, and less "watery." Skim milk behaves badly. Low-fat is OK, but just OK. Save it for something else. Of course, if you keep cream in the refrigerator you can always mix it with low or non-fat milk to make up the difference.
There are four sticks to a pound of butter. A stick of butter is 8 tbs or 4 ounces.
I usually bake with unsalted butter, and add salt to taste. That said, many sweet goods could use a little salt and salted butter is hardly extreme. If you only have salted butter in the house, use it instead of making a panic trip to the store. On the other hand, I'd make the drive for whole milk, if all we had was non-fat.