My only caution about a shed is good quality ones are still expensive, they aren't moveable (rats setting up residence in ours) and you still have to buy timber to and parts to modify (nest box, door, roosting bars, ventilation). Also if its too big, hens can get chilled.
I agree there is rubbish out there (quality and design) but I prefer a flat pack type that can be taken apart to be deep cleaned periodically - especially the joints. I'm experimenting with plastic type for ease of cleaning (and drying) ...