I've now been eating several servings of baby spinach a day. What will all the vitamin A/other stuff in it do for me?

If you Google it you'll find lots of info. Vitamin A is primarily for eye health. I grow baby spinach and put it in practically everything.

Oh, I plan to grow spinach too. I'm getting a late start, so I plan to set it out soon. Is it very hard to grow?

It prefers cooler weather or it will bolt - set seeds. I'd wait until the fall to plant it from seed in your area. If you buy plants now, be sure to keep the leaves picked to keep it going.