I don't have anything to say on the breed, since I have never met a Cocker Spaniel, but I would like to say that there are no guarantees, which I'm sure you know. Even though breeds differ in terms of temperament etc, the individuals within a breed vary a whole lot, too, and you can only hope that you will get a dog that has a sound temperament, good genes and of course then it's up to you to train and socialize. I have two dogs, Australian Terriers, and while the older one is a perfect example of the breed, confident, independent but friendly to all people (including kids), energetic but fairly easy to train, the younger one seems to have inherited a bad temperament, even though he comes from a reputable breeder, one of the best in my country, actually. He is fearful of strangers (and other things), which is not typical of terriers at all, they are supposed to be confident and fearless, and this means he is very difficult to live with, despite good socialization, constant training and of course management.
On the other hand, my sister has a German Shepard, who is such a joy to be around. He came from bad circumstances (that you would have thought to have caused serious behavior problems), but is the perfect dog. He is gentle, very patient and friendly, and is perfect for my sister and her husband and their two very small children.
So I guess what I'm trying to say is that of course everyone should get to know the breed before getting a dog, and buy from good breeders only (or from shelters and rescues), but that you should always be prepared that the dog might not be like what you thought they would be, despite everything you do. This is something I didn't fully realize before I got my second dog. I had somehow figured that since I knew the breed, knew how to socialize and train a dog (I managed to train my first one to be a well-behaved dog, despite some problems we had at first), got him from a good breeder, that there would be no huge problems. I mean I knew those things are no guarantee, but I just didn't think that I (or my dog) would actually be one of the exceptions.
Anyhow, just something to think about.
Luckily most dogs are forgiving enough to come out alright even if we make mistakes. Good luck!