I guess it really depends on what the seller thinks is acceptable. Will you be paying the horse off in full when you pick it up? If not, you and the seller need a contract (and even if you are, you need a contract, IMO).
If the seller isn't sure what an acceptable down payment would be, then I do think that $200 sounds about right.
NOrmally when I sell a horse its $200 down, and if its on payments then I pretty much work to whatever they can afford to send. However the horse does not leave until paid for in full.
If it's just to hold the horse for a week until you can pick it up, I'd say $100 should be fine. It would be okay by me for sure... Why don't you come on down and pick you up one from here, too :bgrin
It would depend on what the seller can afford to finance-- if only for a week should be ok BUT stuff happens & a week becomes a month - then what -- perhaps the seller is selling to pay a much needed bill or maybe he is selling so he too can buy a new horse-- no one can really say what is acceptable but the seller & yourself.
Need to reach an agreement between the two of you.