Its gonna boil down to how many horses, cows, pigs, zebras, antelope and/or hippos you'll ultimately be keeping on the property. Yours, boarder's, yours and boarders, whatever. A couple of horses don't need 10 acres to chew down. Even 3 would be generous. That would mean (as it did in my case), that the unused grassland would deliver 100+ bales per acre, and that's PLENTY for two horses year 'round and sell the rest.
So, I feel there are two ranges of tractor for horse properties: the 15 - 25 hp machine for general lawn and pasture mowing, fencing, ****Flow and picking stuff up. Then there is the 30 - 45 hp machine for the larger endeavor.
Its really just about what your family plans on doing there. You can always buy a second tractor with the hay, lumber, boarding, butchering, or 'fertilizer' profits. Just about everyone around my neck of the woods who thought they wanted to ranch with horses, boarders, lumber, blah, blah, blah, was dreaming. They are dreamers and still doing it. Just can't put it together, money, time, effort, barn, chores, kids, pets, friends, sports, travel, vacations, career, lazy azz.