You need to keep in mind that predators can get in in various ways. Limestone on the edges will help some of the digging problems, but without substantial wire strength it won't keep them safe. Most people tend to use like 2 x 4 welded wire with chicken wire or hardware cloth attached at least to the bottom couple of feet as an added measure of protection. I have strong netting on top of all my pens. My greatest threat is from owls and hawks.
I have coops made of about everything imaginable. I tend to upcycle what I can. The problem with timers on doors is that some birds can get left outside accidentally. I personally never shut my doors on adult coops, cause the pens are fairly secure and I feel the air flow is a good thing. When I get my new building done however, I do plan to shut the chicken doors at night to help keep the rodents from getting in.
One thing I've learned is a raised coop gives you extra square footage. If you have an area under the coop it provides an area of shade and weather protection the chickens wouldn't otherwise have outside. Plus you get an added bonus of more square feet of area for them. I don't like putting the food inside the building so you can also protect your feed from weather by having it under the building. 15 inches or so is plenty to give them extra space and a place you can handle from the outside.
