The biggest worry would be how much they will actually sell for. They might sell for the amount, but a horse is only worth how much a buyer will pay, and if no one is looking for a horse likes yours then they won't sell.
If you are not spending money of food or agistment, and only your time, then maybe do it. The cheaper horse won't really be a profit in hours, but the others may be. But have a solid agreement with your friend and also discuss who is responsible for injuries.
How did he pick them up so cheaply if they can sell for much more?