Better without a perch as some silly first time mums try to take the chicks places that they can't reach and it upsets mum and chicks. This is a good reason to keep them shut in house/shed/run for the first while. Shed with a nest box sounds fine. She will want outside access while she's brooding, but tiny chicks will be fine shut in for couple of days, and then allowed outside as well. Don't think I've ever had any bother of predation with a mother with chicks when free ranging, nor any casualties from other adult birds chasing them.
My casualties have been getting squashed under hens feet (especially in first day or two, when she's scratching for food), getting stuck out of house or nest box and getting cold, or stuck behind in nest box when mum goes out (they understand sloping ramps very well, and may run around underneath. Try to keep everything on one level as far as you can. And getting stuck under upturned feeder that had come apart.
Feed chick crumb and look for poo pasting up their vent; warm water gets it off. There's not a huge about to it.
Make sure everyone can reach food, water and that they are secure. I like to scatter some food on the ground so they get taught to scratch from the start, and I like water bottles just above the waterer, chicks seem to like the hanging drip, they stay clean, doesn't evaporate, no drowning risk and mum can tip it over.
The important thing about setting eggs is to set them all at the same time, so remove everything she's sat on so far and collect or buy in new eggs (or mix of both) and set under her at same time. Also decide what you will do with cocks. I'd set about 10. Good luck x