to your original question, which apples to dry?
You can't go wrong with any apples, but here are some varieties and their characteristics:
Golden delicious: very sweet, tasty, white flesh, a great apple for eating or drying.
Red Delicious: somewhat tough skin, but if you peel them, that is not a factor. A bumpy non-round shape, so they don't peel perfectly like a rounder apple would, but who cares if some peel is left on. Not as flavorful as some apples. They do well in controlled atmosphere storage, so if you are going to dry apples out of season, these are a good one.
Rome: An old variety, has red veins in the flesh, so it looks strange dried, but tastes very good. they don't do well in controlled atmosphere storage, so get them only months shortly after the picking season. Round form, so they peel well.
Winesap: An old variety, more tart than others so its considered a good one for pies.
Granny Smith: a greenish apple, round in shape, sweet, dries well and often has less browning than other varieties
Braeburn, Gala, winter banana, and many other varieties, equally good to dry. There are no bad apples for drying.
Sodium metabisulfite is used commercially to prevent browning of dried fruit and to extend shelf life. This is the same stuff used in making wine to kill the yeast when the right level of alcohol is reached. You can get it at a wine making supply store, or on ebay in food grade for less than $10 for a pound. Using sulfite, dried apples can have more moisture, and not spoil. You mix a small amount in water, and spray it on the sliced apples. It should have no smell or taste on the fruit if done right, and if you get too much on, you let it air out and it will dissipate until you can't smell or taste it.