Unless you have somewhere else for winter hay storage, I would make that area larger. And yes, ventilation is important so adding windows would be idea. More than 1 (one) window would allow for a cross ventilation, especially important during the summer months to keep the heat from building up on the inside. Maybe consider making that half wall a 3/4 high wall or floor to ceiling wall since the hay feeder will be mounted there. I would place the water somewhere other than beside where the hay is fed because hay gets strown when they eat, and the water bucket would constantly be filled with bits and pieces of hay.