Do you have red mites in Australia too?
If not a new wooden chickhouse would be a nice option too IMO.
Personally I dont like to buy (new) plastic for environmental reasons. Also because the cheaper plastics wear down very quickly and the durable plastics are vinyls/PVC and are poisonous and even worse for the environment.
Thats why I bought a childrens playhouse not so long ago. The playhouse is of much better quality than the cheap prefab coops. It’s size inside is about 90 cm * 100 cm and has two windows and a door. If I add a sturdy package box or a cat litter box Im sure it is a great place for a broody with chicks.
To avoid red mites coming in I painted the walls with a DE paste.
I don’t know how long the playhouse-coop will last, but I managed to keep a prefab coop alive for 12 years. I made a new roof, and added some EPDM around the hinges, after a few years. Recently I made a new lid on the nestboxes. Also painted it with a natural preservative a few times. It stands on sandy soil and has pavement tiles /bricks around it against predators. I had red mites in it last year. But managed to overcome the problem within a month, mostly with cleaning and reapplying DE a couple of times. Only a broody was treated with a permethrin spray for birds once and I used the spray in a few difficult cracks.
Edited typo
The hens have had many vet appointments and have never shown evidence of mites or lice. Float tests never indicate problematic internal parasite load.
Aside from ease of cleaning and curiosity, there's no particular reason to buy plastic instead of timber.
The hen will most likely be Ivy, but could be Mary. Either way, she'll need a sheltered environment and the chicks need to be safe from Peggy - the fencing will keep Peggy at bay and shelter can be arranged.
After only one out of six eggs hatched in a raised box at my place, I not confident that raised boxes are what a hen would choose if she had access to a safe nest on the ground. Raised boxes are a human convenience, right? Hens in nature lay eggs wherever they feel safe, like under bushes.
So, I may even use the existing nest boxes on the ground with the floor removed, a table above for shade, and add an extra shade cloth above the table on very hot days. The advantage is that the hens are familiar with their nest boxes already. The boxes are the right size and nice and dark.
Edit: I've thought about using the big coop and I've thought about placing the ground nest under the roost. The big coop keeps everyone very safe but there's no chance for socialising and building familiarity between chicks and hens, and between chicks and the run. Under the roost is lovely and cool but if there's rain, all the hens will need to crowd under, so it can't be fenced off.