Your summer will be less hassle if you scratch goats off your list. The only thing animals need for shelter in the summer is plenty of shade. Are there any trees available for shade? A woven wire fence will hold sheep if it isn't mashed down somewhere. Horses are the most plentiful critters that might need pasture. One hot wire will hold horses. They will need lots of drinking water, but the sheep wouldn't need swo much. I'd require the owners to tend to their own animals. If an animal if not doing OK they should see it theirself and tend to it. Two horses will have two acres shaved to the ground after a month or two, especially if it gets really dry. How much you charge is whatever they are willing to pay. I'd rent it for the summer at a set price, that way if they over graze it and take their animals elsewhere you will still get full payment, which You would be wise to require up front.