You need to figure out how big you want to be, do you want to board or just have your own horses? Do you want to breed, keep stallions, all that will factor into the property you want to purchase. It is easier to get a property that already has the barn and pastures, but then you can't lay it out the way you want, or whoever did it may have laid it out weird. There are rules and regulations you'll need to look up for housing stallions (if that is something you want to do), for using it the property as a business and not just personal use, I.e. Boarding and lessons. For example, the way I understand it where I am, a place that is zoned r-1/ag, means you can have larger livestock, but it can't be a business, like I can't board horses at my place for money, so check into your local rules and regulations. Also, are you going to hire someone else to take care of the animals and property, if so, are you going to offer a place to stay on the property, or is there already a place for help to stay? The type of fencing is also important when looking at a place.
So first off I'd figure out exactly how many horses you want to keep on the property, so you have an idea of what size of property you need, then look into rules and regulations about zoning and such, then go look at some places both pre existing, and land you'll have to build, and see what the cost difference will be in initial purchase price, then ask around the area and see what other barns did, and if they built, how much it cost, so you can figure out what will work best for you. Good luck.