Barn door questions for the carpenter types

I want to build some barn doors similar to this.

I would probably have the windows be oriented horizontally across the the top but this is the general idea. 10' wide by 8' high. Im trying to plan it all out in my head before I start and there are some aspects that I'm having a hard time with. Here's what I have figured  out so far. Pine 2x6 for the main framework, half lap joints at the corners. I think I would probably do a rabbet on the back to recieve the tongue and groove boards on the bottom. 

Here's the parts I dont have figured out. What would be the best, most cost effective way to handle the windows? I could rabbet the back, get some glass cut to size and drop it in, then make some wood parts (muttons? The name is escaping me) to put on the outside to make fake six pane windows. This seems like the simplest. The problem I worry about is if the door flexes too much the windows will break, and the muttons wont work well because I cantfasten them anywhere except the ends, so that part wont be very sturdy. 

The other part I'm not sure of is the joint where the middle horizontal stile (directly under the windows) meets vertical rails. What would be the best way to do that? In the pic you can see that they notched the vertical boards to recieve them, it's hard to say if there is any other joinery happening there. Half lap would be easiest, but I feel like that would hurt the stiffness of the vertical rails. I could probably do a mortise and tenon if I'm careful. Does that sound best? Maybe a mortise and tenon while also doing a shallow notch like in the picture?

How would a smart person do it?

You might be able to buy premade windows that sort of drop in. But either way, the glass needs to be loose in the opening and then you get flexible putty to hold the glass from rattling. 

This will be outside I assume?

I think id be most worried about expansion and contraction of the wood. There are ways to try and align the boards and thus the expansion so it happens in a controlled way mostly in the same direction but I don't know enough about it.

In reply to Robbie (Forum Supporter) :

Yes, it will be outside. The windows being loose does make sense.

As far as expanding and contracting goes I wonder if the tongue and groove panels should be floating in the door also.

For windows, go to your favorite lumber yard or big box store website and search for 'barn sash'.  They will be the least expensive premade windows you can find.  Here's an example from Menards.  https://www.menards.com/main/search.html?search=barn%20sash