have you discussed your concerns with the breeder? what was their response? If you are feeling hesitant that the puppy won't fit in or isn't what you are looking for, I would recommend going elsewhere. Does the breeder have a reason that they think this is the puppy for you? Or is it simply the only one left and they have to sell it to someone??
I would think that, given your concerns, the breeder should be willing to work with you. Even if it means waiting on a different litter to get the RIGHT dog.
ETA: another thought. Have you had "one on one" time with the puppy? or only in a large group with the others in the litter? that could possibly make a difference in his interactions with you, but I'll let someone more knowledge tell us if that is true
