Also, make sure that you have your OWN vet to do a pre-purchase exam on the horse. I would never go through with a sale until the PPE is completed by a vet that I trust. Some people will suggest that their vet do the PPE, and every now and then you'll get someone who's honest, but I've heard of many occasions where the seller and vet had an under the table deal and the buyer got stuck with a horse with some serious health issues.
The first horse I had vet checked did horribly. He was dead lame that day, and the vet said that based on her observations he will likely be lame often and unpredictably. The seller was going to try and sell that horse for $3,000, but talk around the barn revealed that they've been trying to give him away for months. >:(
My current horse, however, had a nearly perfect vet check which is exactly what his previous owners were expecting. Since Sock was living quite far from my vet, I asked him for a recommendation for a good vet in the Modesto area to do the check.
Although it's not a warranty, a PPE done by a vet you know or who was recommended to you by a knowledgeable horse person can give you some peace of mind when buying. If the current owner refuses to let you get the horse checked before you buy, I don't care how sweet that horse is, run. FAST.
I hope all goes well! Just sharing my recent experience from one new horse owner to another. It's a wonderful thing when you find your perfect horse. :)