I don't know about with horses, but I have done payment plans with other items. Just make sure you have a through contract in hand which states what action you will take when. I would probably do a X amount per month, and if not paid in full by X date, then u go get the horse kinda thing. I have one case where somebody bought something from me, then went through some hard times (divorce) and still hasn't paid me back, but this is mainly because I gave him quite a bit of slack, and honestly its not enough for me to care too much about. How strick you want to be about payments is totally up to you. If you want to put in their contract that if they are 10 days late they have a fee, like a lot of landlords do, then go for it. If you just want to put a 'you must have it paid off by this date' in there, then do that. It's up to you, just make sure its all written down and signed.