I can't see any photos, but here is my experience with cracks:
Thin ones do not usually pose a problem, some will even heal with time. A crack that goes all the way to the coronary band will not heal. Horses with crack sometimes have a tendency to develop more cracks, usually due to a nutritional deficiency (in my experience) or simply an inborn tendency,
Large cracks that do not go all the way to the coronary band are very treatable. A mare at my barn had one recently because of a rock or abscess (according to the farrier). It takes some time, but the hoof will grow back. She does have other cracks in her hooves that cannot be done away with, but these do not bother her, and she is developing less of them as we get her on a regular trimming schedule and a balanced diet.
So don't let minor imperfections put you off a good horse! Be aware that this horse might develop more serious cracks in the next few months, though these should be less frequent with the right care and food.