Horses have a less efficient digestive system than cows, so weed seeds pass
through them and can be germinated, so compost the horse manure well or you'll
just be spreading weed seeds.

The uncomposted straw will rob the soil of nitrates initially, but once the
straw breaks down the nitrates will be released again and available to your
plants.

I'd suggest you get everyone together and build a compost container from old
railway sleepers, or something similar if at all possible. This would give
everyone a large heap to place compostable waste on and ensure the heap got
nice and hot to kill off and weed roots/seeds. Most small heaps just don't get
hot enough, and are often a lot slower to break things down into usable
compost.

Just my opinion, FWIW.

Dave.