The Cocker Spaniel breed has had over breeding troubles because of a Disney movie (among other factors). There are genetic lines within the breed that have to characteristics that you have mentioned, but there are also many lines that are sweet as sugar. Unfortunately its hard to say what the genetic predisposition is for the dog you are considering adopting. That being said, they may have genetic tendencies, but proper training can help offset their genetics quite a bit. It sounds like you take manners seriously will make sure the little lady will behave appropriately.