You just need to spell out that the payment is due on a specific date each month, if the horse is not picked up by a certain date, X amount will be charged (per day is usually easier than a monthly rate, makes calculations easier, you may choose to wave this at your own discression if the shippers schedule is booked or for other reasons, but it is best to have it in the contract as a safety net) sometimes buyers will drag their feet when it comes to picking up a horse.
INSURANCE it is a great thing. It would be awful to have a horse almost paid off and have a slew of medical bill, or be a seller stuck with the bill. I understand in some instances it is not practical, like insurance on a low priced animal, I am guessing that is not the case if they are going to take 6 months to pay off the horse. If you are not going to require insurance, you need a DETAILED list of who is responable for what (Maybe you will cover feed, hay,wormer, and hoofcare, they would be responsable for anything that would require a vets service).
I have gotten horses on payment and greatly appreciate when sellers have worked with me. I have sold horses on payments too. It can be a good experience when all parties involved know what their responsabilities are and uphold there end of the deal.
Carolyn