A lease w/buy option generally has the purchase price set and the purchase time period. It also has specific language about the buyer defaulting, the standard of care, and other requirements.
A trial is negotiable. I wouldn't allow one off my farm because of the risk of the horse getting injured in a new setting. However, others do allow trials. Like a lease option, it's all entirely negotiable and that includes the deposit/purchase price.
Here is a site that offers legal forms for all manner of equine transactions. Sure you have to pay, but it should be worth 25-50 bucks for the peace of mind that you and the seller are well covered by a solid agreement.
IMO since you don't have experience with negotiating these agreements, it will be better than piecemeal info from COTH posters.