Have you had any contact with the seller yet? If not, just email and see what they were thinking as far as price.
I always have horses for sale, and I recently sold some as best offer - I got offers from $50 to $900+ per horse. It really just depended on the person's situation and I understood where every offer was coming from.
This isn't a science report or debate, so you don't have to include references in your offer. It is good that you did your homework however and have an idea of whether the horse is in your price range.