We have three horses and are close to Benson, as well Chiltern Microlights. The horses take very little notice, though they were a bit spooked by the Tornado in full re-heat at about 1,500' one day. Helicopters at less than 200' are a bit of a surprise as there is little noise in advance, and then it's loud, and there's the key; they hate loud noises close up with no warning, but if they hear the sound build gradually then they will be fine. An approach into a stiff breeze might thus be more of a problem than them getting plenty of warning because they are downwind of your approach.
I think the low overfly is a good plan, but I'm less sure about the prospective tenants being OK with it. You may have to do some demos, and bear in mind that some horses are naturally more spooky than others. Thoroughbreds can be very edgy, while a good Irish cob might not even bother.