I am going to recommend you buy a pad of graph paper and sit down and draw out what you think you need/want. Google cattle sheds, horse sheds to get some ideas. A building with stalls is quite costly, whereas horses are happiest milling about in a semi open building. Check lumber prices for 2x8, 2x10's, roof and wall sheathing, shingles. Draw everything as best you can to scale. You can make changes on paper much cheaper than once construction starts. A few corral panels enable you to set up a temporary stall if needed without building permanant stalls. Horses seek out shelter to escape the bugs, not the weather.