A feed room perhaps, with built in or added feed bins. Dont forget to mention what kind of bedding you would have.
For your plan i reckon draw up bits of it and describe the others? For example, say your talking about the stalls. Describe the bedding, layout, materials it's made from, etc.. Then draw a birds eye view (labeled) of it. Like draw a birds eye view of the arena and round pen, paddocks and barn.
For the essay bit just focus on on bit at a time, like this arena is ideal because..blah blah blah.
This sounds like a really fun project. The best thing about horse related projects is, you can write a wrong fact and teachers wont even know (not recommended tho). I confused the hell out of my english teacher when i wrote a story about horses, she emailed me asking what did agistment, farrier and diagonal mean.
If I am not riding I'm thinking about horses, and if I am not thinking about horses I'm riding