I hope the slide out tray wasn't a deciding factor (they simply don't work well - the poop gets knocked out if you try to pull them out). For predator safety, it's best to fix the floor in place.This is the one. I can confirm it's better built than most other prefab coops. Compared to the other ones at TSC, it looks much stronger. It says that it has 15mm walls, which should be ok from what I can understand. It also has a good clean-out tray and more ventilation than the current one. It would be a fixed coop. I'm aware of the few 'predator proofing' modifications I'll need to make.
https://www.tractorsupply.com/tsc/product/petmate-superior-construction-chicken-coop-70401d
Are you sure it wouldn't be ok for three bantams? I'd probably go with a silkie, a barred rock bantam, and an easter egger bantam. I thought that bantams were good with 2' in the coop and 5-7' in the run? I could probably find a bigger coop, but it'd be a few weeks longer of them in the current coop if I did that. Thanks!
You will need more ventilation. I don't see any - the door doesn't count unless you leave it open at night (and even then doors do not offer optimal ventilation due to the fact that they're low), and the window looks to be plexi? It says there's a ventilation hatch somewhere but I'm not seeing it, and ventilation should be up high.
My suggestion if you absolutely must buy this is to convert the entire unit into a coop only (if you want instructions for that, let me know!), and then add on a larger run. Personally I'd suggest a minimum 100 sq ft run no matter how small the flock is. It's not just a matter of space that the chickens need to move around in, but also the more space, the less cleaning you need to do, the less poop and odor builds up, the easier it is for you to go in and clean and maintain.
I lived with a prefab for about 2 years and I admit I did not clean it as often as I should've, because it was so hard to bend down and move pieces around.