I lost more than forty chickens at one point while I was away working in Florida. Coyotes came in and wiped them out. Another pieced-together pen later lost about thirty birds. Those are not abstract warnings once you have carried out the birds.
A pen can look functional and work for months while still having one weak edge, loose corner, easy latch or unprotected opening. Predators have more time to find it than you do.
Build the pen for the animal trying to enter, not for the person looking at it in daylight.
The order matters
Finish nighttime containment before expanding the flock. Check every seam from ground level. Protect the top and the bottom. Make the gate and latch as strong as the walls. Then decide whether the surrounding fence and working dogs add enough layers for your actual predator pressure.
The replacement pen does not have to be pretty. Salvaged kennel panels, old metal, chicken wire, two-by-fours and barn wood can make a better system than the first one if the weak points are deliberately removed. Perfectly imperfect is fine. Function comes first.
The flock should not pay for unfinished infrastructure
Buying birds is the exciting part. Containment is the part that keeps them alive. If the pen is not ready, the livestock purchase is early.