Big coops can be great if you have a few chickens and don’t free range and don’t plan to add to the flock. Once built your limited to the number of chickens you can keep by the size of the coop. Sure you can, and people do tack a bit on to extend it but there is imo a much better way.
That bit of a leaning shed has had it’s day. Wooden posts buried in the ground always rot. You could spend lots of time and money trying to make it look decent and be secure and be doing it all over again when the uprights rot out.
Rip it down.
Have a look at my coop page. You could probably build these or similar for about the same costs as renovating the shed in your pictures.
If you built coops like the ones in my coop page maybe twice the size they could still be movable with a fork loader. Coops twice the size of thos in my coop page would house 15 chickens if they free range.
50 or more hens in one locality will strip the area bare in next to no time, free range, or not. A major factor in the health of your flock is the condition of the ground they live on.
If you plan to have roosters then multiple coops makes this so much easier.
If you want more chickens you can build one of those portable coops for a couple of hundred dollars.
With the 4 live in coops I have I can house up to thirty chickens.
Think along the lines of lots of small rather than one big would be my advice given you’re starting from scratch.