At 400 feet, you will smell it at some point. It's inevitable. But it shouldn't be a real big problem. If it becomes a big problem, country life probably isn't for you.
I knew a farrier who had a boarding barn. It had been there for 30 years before some genius bought the land adjacent to build a subdivision. People paid good money to move in, and then complained about the smell, to the point of trying to get zoning laws changed to shut him down. In a farming community, that went over like a fart in church. The stable was there first. What did those people expect? Horses pee, poop, and procreate outdoors. It ain't pretty, but it ain't changeable.
OP, even if it does smell, if you spend any time getting to know those horses you may find that you enjoy interacting with them so much that the smell won't matter much...