The coop we have planned will be approx. the height you're thinking of: from 4' to about 5.5' at the peak. Because we're trying to minimize the footprint in the yard, we're raising the coop 2' over a portion of the run. So, I'll still be saving on the cost of materials, but I'll still have great access to the coop for cleaning without stooping (there are a few cleanout doors that will be waist height to allow a wheelbarrow underneath to scoop shavings into). I'll also be saving on the cost of roofing that portion of the run as it's already protected by the coop.
As for the external nest boxes, we decided they wouldn't work in our climate. We also see long stretches of freezing weather and had a few blizzards last winter. So, we'll still have egg collection doors mounted on the oustide for easier access, but the nest boxes will be mounted inside. As I plan for several doors on the coop (clean out, egg doors, access doors for waterer and feed), we're planning to insulate them and surround with weather stripping to minimize drafts and heat loss. We'll also be insulating the walls, ceiling, and the floor since the coop is suspended.
Good luck!