Hon, I admire your intentions, but I would never entrust the care of my horses to you. You can read all the books in the world you want, but it comes down to real-life experience. Learning long before you have your own horses is best, learning with the help of others as a first-time horse owner is forgivable, but learning the basics while you play caretaker of someone else's animal is unforgivable.
Have you studied nutrition in depth? What do you know about starch, fiber content, protein content, sugar and legumes? Do you understand the regimented feed plan necessary for an Insulin-resistant horse? Or what feeds put weight on malnourished horses without adding junk like sweet feed to their diets? Comparing horses to cattle is very naive and can get you in a lot of trouble. For example, cattle can consume hay with some mold, but this is incredibly dangerous for horses.
What do you know about topical antibiotics, pain relievers, and anti-inflammatories and their uses? What dewormers are not safe to give young horses, and what dewormers kill encrusted strongyles? How much epinephrine should you have on hand whenever vaccines are given? What is normal procedure for emergency care for tendon problems?
What kind of insurance will you have on the property? Will the costs still allow you to make a profit? In the boarding business, most people barely break even. The majority of boarding barns make their income through lessons from qualified instructors. You NEED to have money saved up to cover the months that you don't make any profit.