As long as you buy your pup from an experienced reputable breeder, you should have no problems whether you choose a solid (self-coloured) or a particoloured puppy. A reputable breeder will be more than willing to show you their other dogs so that you can see for yourself that they have friendly, happy temperaments. A good breeder will also take the time to socialise their puppies properly (important if you want an outgoing puppy) & will do the recommended health-screening tests such as annual eye-testing & hip-scoring. It is better to contact a breeder via a recommendation from one of the Cocker Breed Clubs, a full list with contacts can be found
here. Avoid commercial kennels selling a number of breeds & be very wary of adverts on free ad websites or in free newspapers. As far as price goes, this varies with location but £550 is reasonable for a KC reg puppy from a reputable breeder (whatever the colour).