Hallo Amber,
as someone keeping horses, much of their need is similar to donkeys needs, but I wouldn't assume that you'll be able to feed them entirely of your property ( even if you plant nothing but grass). Your regional AG Office might have data in how many acres are needed for 1 animal, you should still get 2, in my opinion a decent rescue shouldn't give you one to be kept alone. There is always a reason why they land in a rescue, often age and the associated health problems... Read read read before you commit!