you can definitely make the vet care and farrier work the responsibility of the horse owner. You should investigate your state's liability laws as they pertain to horse activities, and how much your insurance will be.
It can be a huge pain in the neck to live on the property, with people wanting to come at all hours of the day and night. Also, do you have horse experience? I wouldn't generally recommend that a "newb" jump into a professional horse operation right off the bat, there's a LOT to learn.
Who is going to be responsible for the chores? If you are at capacity, we're talking about 21 horses...which is...a LOT....of horses to care for by yourself. We have 5 horses, and morning chores (with stall cleaning) takes a couple of hours. Add in catching, turn out, blanketing, fly spray, feeding twice a day....and you're talking about a near-full time job for one (or even two) people. I worked for a while on a hunter farm that had a 16 stall barn, and let me tell you, that was the HARDEST work I have ever done, and I was there from 7 am until I finished in the evening, usually around 7 or 8 pm. I haven't even considered the work that would go into simply maintaining a 34 acre farm, fence work, weed control, pasture care....it's a huge operation, and you will most likely need an employee or two.
Also, you can kiss goodbye any chance of leaving town, even for an overnight trip. The horses will need someone experienced there with them, looking out for them, and caring for them. We usually hire a professional farm sitter to watch our horses, and it runs over $100 a day for that, so it doesn't happen often, lol.
And back to the experience bit. Part of what the horse owners are going to be paying you for is watching out for the health and well being of their animals. I don't know your level of experience with horses, so you'll have to answer these questions for yourself...do you know the signs of colic? What would you do in case of a catastrophic injury? What if the owners aren't getting the routine or preventative care that their horses need...you can't just let them stand on long hooves, that's neglect. You need to have a contingency plan in place for these (inevitable) situations, and be prepared to shell out the money to care for the animals yourself if the owners don't.
Also keep in mind that the horse market is in the toilet right now, with the economy the way it is. You need to think about the eventuality that SOMEONE is going to stop paying their board...and although you have collateral against the board bill (the horse) chances are, you're not going to get diddly squat for a horse that the owner would simply abandon.
I wouldn't start a boarding operation with the expectation that I will be able to survive off the proceeds. Sure, it's a possibility, and many people do, but many more start up without a clear understanding of the real expense and headache that goes into it, and they end up shuttering not too long after. If you want to have a couple of boarders, go for it...but if you have the mentality that it's a HOBBY, and not a real moneymaking venture right off the bat, then you won't be so disappointed

That way, if your business DOES take off, you will be pleasantly surprised
