Horse remains your property until the final installment
Horse to be fully insured for market value including loss of use for the period of the loan
I'd take a 20% non returnable deposit fairly soon into the loan period, to cover you for any lame horse that might be handed back at a minutes notice
Consider what activities you don't want the horse to do during loan (ie anything with a high risk of horse getting injured)
All tack equipment to be returned in good condition if not included in the sale.