I think the title question isn’t phrased quite right.
It would be the right question if you were the buyer. As the seller, you’re real question is: how much should I discount this colt for a fast sale, and how can I tell if this buyer is finally serious?
We can’t answer the first question without intimate knowledge of your region, breed, and pedigree and even we’d all disagree
And as far as the buyer being serious, hard to know.
OP what you need to secretly decide is your absolute minimum figure that you would sell for, and keep that in mind as you play negotiations with her.
Buyer will want to feel she has a bargain.
Payment plan would make me nervous. What if she only pays part of it and disappears the colt? Get her to save up and pay in full.