I worked on a horse farm for about 5 years, it was one of the earliest jobs I had (and unpaid - basically I got a steep discount on lessons and got to be around horses).
Taking care of even a single horse is enormously labor intensive. We shoveled horseshit twice a day, at least, sometimes more often. Horses eat something like 20-25 pounds of food a day, wash it down with 10-12 gallons of water and produce a similar amount of shit. This all has to be removed from the barn and small paddocks.
You will need to store this food. That means a place secured against horses AND vermin. You will have mice and rats. This is why horse barns almost always feature semi-feral cats, they’re there for rodent control.
Horses need their feet cleaned. They need their coats brushed. Their feet need attention whether barefoot or shod, and if they have shoes that means you need a farrier to come out every 6-8 weeks and that’s assuming there are no problems. And there will be problems. Horses are hard on their feet and a horse with bad feet is a dead horse.
OK, you say, we’ll make the owners do it. Sure. What about middle of the night emergencies. You’re talking about an animal that can die of a stomach ache (seriously - they call it colic). You’re also talking about a big, strong animals that, if frightened, can injure or even kill a human being - can you handle some sort of emergency with one? What if one escapes? Horses jump, they can knock/kick down walls and fences, and the farm I was at one winter the snow drifted and packed down/froze in such a way it basically made a ramp over the fence at one point and suddenly there were horses wandering around the neighboring areas that had to be rounded up.
What if the owners stop coming around, neglect or abandon their animals? Several of the horses at the farm I worked at were humane society rescues and usually what happened is that someone acquired a horse without a really good understanding of the expense and care involved and neglected it severely. Please, don’t be that person. The problem is, if you board horses you will sooner or later encounter that person - what will you do?
What if the animal(s) get sick? Who calls the vet, and who pays? Either the vet comes to the horse, or the horse goes to the vet but for the latter you need a trailer and a sturdy truck to tow it, not to mention the problems of getting a skittish animal that doesn’t feel good on and off it. If a horse dies what do you do with the carcass?
Horses are basically toddlers and like toddlers they put things in their mouths, wreck things, and can be destructive… except these toddlers are enormously strong (we measure work in *horsepower *for a reason). Who is going to fix the buildings and fences? You have to “horseproof” their living areas just as you “toddlerproof” a home.
If this was a turn-key business with reliable employees already running the show that might be one thing, but it sounds like you’d be getting into this without employees, yet you say you know nothing about keeping horses.
As property owner you can’t escape liability. You can and likely will be held accountable for neglected or abandoned animals. If animals get loose you will likely be liable for any damage they cause, and for any injury the animal suffers. Being around horses carries an inherent risk - someone gets hurt or killed on your property you’ll likely be on the hook for that. Someone WILL get hurt - fall off, kicked, bit, whatever. We had idiots who snuck onto the property and tried to ride the horses. We had idiots who fired guns or did other things to deliberately frighten the animals and make them run - never mind panicked animals might get hurt. We had one suspicious barn fire that resulted in a pony dying of a heart attack after getting wedged in a fence while trying to flee - a dead horse is enough aggravation, one tangled up in a steel fence is even worse. You either need some heavy equipment and a big trailer to move it or a chainsaw and a strong stomach. And you’ll still need a trailer.
This is not something for beginners to get into unless they have a lot of help and a lot of money. Horses are like boats or airplanes - a hole you throw your money into. Yes, some people can turn a profit but it’s not easy, it’s an enormous amount of work, and most folks can’t pull it off. It would be one thing if you were acquiring a pet horse for yourself but running a business like this not for amateurs.