Totally depends on your climate. If you're somewhere that doesn't have many miserable days when the chickens will want to stay indoors most of the time, AND does not get very hot and sunny in the summer, then it would probably be fine, with adequate ventilation. If you're somewhere with long cold winters or long very-rainy spells, a larger henhouse would be smart; if the coop will experience hot sunny summers, an opaque roof would be smarter (for a small coop like that, opaque plastic might be a bit better than metal if you have to choose between those two; or insulate under the metal). Generally transparent or translucent parts to let light in are much better situated in the WALLS than in the roof.
Three nestboxes is overkill, all you need for 6 hens is 2 boxes, or even just one size-and-a-half one. If the coop will be tall enough, it is much better to put the boxes INSIDE the coop (14+" above the floor litter, so the hens can still use the floorspace under them) with an outside access hatch to them if you want outside access. This is far easier and gives you all of the advantage of exterior boxes without ANY of the considerable disadvantages of them.
Good luck, have fun,
Pat
