I am glad you have lots of time to check him out. Ask the owner exactly what is "wrong" with his leg and if a vet tended to him. Keep in mind what you want to use him for. If you are going to mostly trail ride, you probably don't need a speed demon. But you still want a horse that walks fast enough to be fun.
My mare has lazy tendencies but is easily moved forward when I choose, kind of a go-with-the-flow type. She was really pokey when I tried her out, but she had NO training at all, had just been sat on a few times and let to walk as she pleased.
If this gelding has been trained, find out how long, what kind and what he is supposed to know. Talking to his trainer instead of taking the word of the seller is a plus.