5 acres is the total property? So remove at least 1/2 acre for the house and whatever lawn and driveway you have.
I would not at all consider any LGD breed for a first-time dog. They really do require very careful people socialization so you don’t end up with a dog who disobeys you (common with those breeds) and attacks someone invited onto your property as their way of defending it.
5 acres really isn’t enough - they WILL be leaving, one way or another.
If you have just 1 horse right now, and that will be Horse 1 on the property, then right off I’d get a mini donk who is already known to dislike “visitors”. Keep in mind if you end up with a pet dog of your own, donk may not take kindly to him either, and that’s ok if what you’re really after is a donk who will keep fox and coyotes away. You’ll just have to never let your dog in range of donk.
4 acres is the most pasture you’re going to have, by the time you take into account a shelter, dry lot, manure management area, and hay storage. The more different animals you get, the more variety of paddocks you’re going to need. Horses can live with sheep and goats, but you’re still talking about quite small acreage, and they’ll all have to be off grass when it’s too wet.
That means a few smaller paddocks, and that means the more animals, or the more variety of animals, the more separation they’ll need, or you raise the risk of fights and injury.