All merles can have health issues, it really depends how the pigment is placed as it is a random spotting gene. It is usually hearing and/or eyesight that is affected, which should be obvious in a 6 month old pup. So check and ask if he has been tested for deafness, and see how his eyesight affects him.
Many merles are perfectly healthy, it is usually the offspring of two merles aka 'double merles' that have serious issues, and no responsible breeder would do that mating. So do ask how he was bred, the colour of his parents etc.