Prefab coops can be improved upon however they still aren't desirable, even the birds will refuse to use them, they are just too small for all involved. I'd definitely consider updating a wooden shed if possible.
If you've been in the chicken keeping practice for 3 years and are looking to expand, now is the time to find a used shed and convert it into a proper coop. You will never regret it. I suggest visiting local shed builders and find out what they do with old sheds that are sometimes removed from properties. You might be able to have one delivered to you for the cost of the delivery fee only. It can't hurt to ask.