so pretty much take the horse to the vet is the pre exam? And about how much might that cost?
Will this be your first purchase of a horse? Do you currently have/know a large animal vet?
For large animals, vets typically come to you, although I have trailered horses to the vet's office before. The vet will charge for the farm call, and that cost depends on your area and how far he/she had to come. My last farm call, about a month ago for annual shots/coggins test draw, was ~$30 US. IME, most horse vets do have a rate for pre-purchase exams that varies with the depth of the exam. It can range from something simple (lameness evaluation, pulse/respiratory check, eye/ear check) to x-rays and blood tests. It all depends on how sure you want to be about the horse's physical condition and how much you're willing to spend. A more expensive animal, or one intended for more intense work or competition, should have a more exhaustive PPE. A less expensive horse, or one intended for lighter use, may only need a simple evaluation to rule out major issues, like visual problems or hidden lameness issues. $1-200 for a simple PPE would be my estimate for my area, and is a small price to pay if it saves you from several thousand dollars worth of vet bills and sunk costs.
If you don't have a large animal vet already, you'll need to have one soon anyway if you're buying any horse. Start calling around and ask who is taking new clients, and inquire about pre-purchase exams and fees. Ask any trusted horsey friends or neighbors who they recommend. Do use your own vet for the PPE, DO NOT assume that the seller's vet is above board, or simply accept the seller's word that the horse is totally sound. It isn't fun to expect the worst of people, but a horse is a huge cash outlay, and you need to cover all of your bases to avoid getting stuck with something you don't want or can't afford to deal with.
The vet will not tell you whether to buy or not. All he/she can do is tell you what the horse's medical condition is. You need to have an idea of what you can and can't live with in terms of the results of the PPE, and use that exam as a tool to further inform your decision.
Sorry for the novel on that, but there's a lot to buying a horse to keep in mind.