Hi, I'm new here also. I have had my chicks for 6 weeks. I built their coop and run from a mix of recycled and new material. And I can already see I'll need to add to it! So yes build bigger than you think you need, chickens need room to grow! And they grow fast! Enjoy them and spend lots of time with them, they will start to come running when their hear your voice, expecting treats and pets!
As for feed, I'm no help on that. I am looking for a organic feed for my birds, my local feed store doesn't carry a great selection of chick feed and no waterfowl feed at all. I do supplement my chickens with garden clipping and grow oat grass for them. I simply buy a 50lb bag of deer oats and sprout them, plant them and then harvest and feed it to the chickens and geese. They love it! They also get Brussels sprout leaves and turnip greens from my neighbors gardens, she loves feeding my birds from her garden, says she don't like wasting. Lol. So if gardener live near you, you an ask for some of their clipping for your babies, just don't over feed them treats while they are young. There are plants that are easy to grow if you like gardening that chickens love to eat. Nasturtium is a great flower for both beauty and is edible and chickens love it. I plan on planting some around my chicken run for them to have shade and a treat. Lavender is another great choice, and some herbs are mildly medicinal so letting chickens graze on them can be good for their health. Just be care as some plants like lantana, and wisteria (2 plants I have in my yard) are poisonous to chickens, dogs, cats, and people. Depends on how green you want to go.
Welcome and I hope your babies bring you as much joy as mine do!