I would be very, very careful. A perfectly healthy, well-mannered horse will be a mile away from your minatures - A malnourished, neglected, who-knows-what-handling, stallion - Now that will be about as far from your miniature as you can get. Even experienced horsepeople get injured by horses such as these.
Also, as horses put on weight and get healthy, they often have drastic personality changes - They start feeling GOOD, so start challenging authority.
Is it possible to call Animal Control, or a local rescue? If you have the money to buy them, you could by them and ask a rescue to take them on while they are rehabilitated, and do an assessment on their personality and suitability for you - then you could take them/it home, or find a more suitable home. They have the resources and the experience for this sort of horse.