The site looks perfect, they'll appreciate the tree. I wouldn't worry too much about clearing it, the girls will enjoy doing that for you!
My pen is about that size, possibly a tad bigger. They can't have access to the garden because of the dog, but are more than happy in their pen. I've used weldmesh to construct a walkin enclosure, and buried at least 12 inches around the edges. Initially roof was also weldmesh. since then, I've roofed the whole thing with polycarb because it turned into a mud bath when it rained. The girls are incredibly good and enthusiastic diggers, and even with wood chips down, it became very messy, and they were just miserable. Hens don't like being wet. The girls are so much happier in the dry.
We made our own coop, as many on here have done. Really simple. Whatever you do, keep in mind all the time, "How can I clean it", and avoid anything which might harbour redmite. We designed our coup so that the majority of joints were external. Inside is a clean and simple box.
Make sure coup is well off ground if poss to avoid rats.
And consider auto pop hole opener. Worth the investment.
Re feeders etc, would recommend the green bottom ones. Started with the red bottom ones, but wasn't impressed. With 8 hens we have 2 x 3kg hoppers plus an 8 ltr water container. More than adequate. Also they need access to grit, so a little container for that too.
My 8 are all ex batts, and I've had them 3 months. As a complete newbie, I've been very lucky in that they haven't had any serious health problems, and are just a complete joy. They've blossomed into happy, healthy chickens, and it really does your heart good to see how they enjoy life here after those awful cages. They are busy all day long, and delighted to see me when I go down the steps. Incredibly friendly and inquisitive. And despite being in retirement, they produce at least 6 eggs a day, regularly 7 and occasionally 8.
But, I do take the point, that ex batts can be a challenge if you are unlucky and get ones with problems. I do realise that I've been very lucky. So hope you are too which ever way you decide to go.