Just a few comments from a former horse owner and occasionally participating in Humane Society Rescue projects (where to stash seized neglected horses until the Legal Agencies sort it out).
You need 'loafing sheds': usually a two place 3 sided building with the opening away from the main weather direction. This is where they will sleep, snooze, nap, nuzzle, get out of the sun and hide from you.
Then you need a 'barn': a place where they are contained or stashed until the vet, farrier, trainer, or rider is ready for them. Catching them out in an open pasture sounds like fun, but it's not. This is also where we put them in inclement weather (freezing rain, tornadoes, mosquitoes, sore, hurt, being fed individually and cleaned.
The barn also serves as a storage area for grain, clothes, saddles, sawdust (for the stalls) and baled hay (which you may need when they have chewed down all your pastures). The barn also needs a wash stall (you don't want dirt and poop on your britches, do you?), maybe a toilet (got trees around?) sink for cleanup, cabinets for creams, salves, shampoo, bug spray, leather conditioner, (and also some stuff for the HORSES).
Just in case, a foaling stall is desirable by girlfriends, wives, relatives, and boarders. Its a double stall with a removable partition so a Mommy horse and her Baby horse can have some room to get to know each other. Nothing like a refrigerator for drugs, ice, and pop or brews. You need a lockup area too. A room with a lockable door.
Keep your design(s) open to the notion that someday, you won't have horses but may have some nice tractor equipment to store: mowers, cement mixer, snow plow, water tanks, windmill parts, lumber, extra roofing shingles, another tractor or two, a drone repelling device and a workshop. A slab to hold a horse trailer out of the mud is also a good strategic item.
All my barns and sheds are concrete block with concrete aisleways. I hay off the pastures now and sell the baled hay to neighbors. If you have excess pasture land, consider baling equipment. Its a LOT of fun once you get the kinks worked out. Having your own hay also keeps the horses healthy because you know the content. There are a lot of poisonous weeds around now and that can be expensive.
Nothing like a boarder or two to help with work and or costs. A 4H prospect is a pleasant experience.
Don't get me wrong, you don't need all of this. But, its just work and a little money. With the wonderful construction skills you have demonstrated, I'd imagine this will be a very enjoyable investment for your family (and your IRA).