Agree with comments so far. My biased comment: The whole point of a metal roof is to have no problems. I'd do the sheltered, fixed windows up high. Easier to fix, less problems if they do leak, you still get light. Another option that I'll be using for the next outbuilding is having the top panel of the garage style door so that it has windows. That will let in enough light to let you get in and open a door for more light.
One thing I didn't gleam from your post is are you trying to get enough light to be inside and get work done, or enough light to get in, get something, and get out?
Pete