What is the best piece to smith at lower levels, as far as cost of materials vs amount of training received. For example, should I make iron helmets or iron armor? Or does it wind up being about the same?
If you are going to buy materials, make them and sell it back to the blacksmith, iron dagger is cheapest among the three. But if you are going to buy materials, make them, improve them and sell it back to the blacksmith, iron armor comes out on the top, but by very very small margin.
If you hunt and have lots of leather, making and improving leather helmet is optimal. It makes slightly less money than making leather armor, but gives you more experience.
Buying training from a trainer is more cost effective than buying materials from the smith and making iron daggers until level 50.
If you find a daedric armor in the wild, it is most likely worth it to pick it up, buy an ebony ingot, improve it, and sell it back for free experience and extra profit. But buy the time you find one in the wild, you will probably have maxed out your smithing already and may decide that extra hassle of carrying it back home is not worth it.