Sometimes it the little things you miss the most!
these items are needed too:
Scissors to cut open hay, manure fork, wheelbarrow or muck bucket and wheeled holder, scrub brush, buckets for food and water, slow feed hay nets for overnight, big broom, heavy duty hoses, soap to wash yourself with, towels for wiping everything, non-latex gloves, first aid supplies for the horses, metal cans for feed (I use metal trash cans), bug spray for ants, wasps, etc., shovel for digging up weeds, and maybe a barn cat
Also spare wire for fencing repairs (this can happen sooner than you might expect) and a fence tool.
Good lighting is essential and must be where a horse can't reach it. It is super handy to have big spotlights on the outside of the barn too, for those dark evenings.
A designated place to dump the manure. Storage for shavings and hay. IMO you need space for more than 100 bales. There are 365 days in a year, and three acres is not enough to support two horses. I had three horses on three acres and I used 400 bales a year, minimum. Two were very easy keepers.
Very nice to have cover in the pasture too. A metal carport works great especially if there are no trees.
Last but not least, trailer storage. Anyone with their own place definitely needs a horse trailer and a place to put it.