Research on a particular breed is important - it contains a lot of information that could influence your decision. It is not prejudice to say, for example, that a Border Collie is bred to be a high-energy working dog, and that leaving one alone in a small apartment for a day will probably just lead to trouble. Research could include contacting breed rescue groups, to see the usual reason given for dogs being surrendered to rescue. Cold, hard data.

No good factual data would say "Dobermans are bred to bite. Goldens aren't." It would include other information, like Dobes can be protective and one-person dogs, and what to look for in a puppy if this is - or is not - what you want in your pup. It would include information like "If you don't want to be bothered brushing a dog, a Golden might not be the dog for you." and "A Doberman's coat is sleek and low-maintenance."

How could this be regarded as prejudice?

Just the facts, ma'am.