The first requirement would be lots of trails,
Second, good pastures and individualized feeding for fitness,
Third, easy trailer parking and access,
Fourth, having a group of endurance riders would be a plus. It would make it easier to have help training greenies, plus sharing trailers to go to events, and just having other like-minded riders to discuss issues with. It would be great to have a group to train with, for safety & fun.
Oh, a good farrier that comes out once a week or so would be great too.