Great danes help please?

So my family and I have wanted a great dane for quite some time. We have done research and it the end decided to go with a great dane in rescue because there are so many of them out there. We worked with our local Great dane rescue to find one that met our needs, and found one. She was big a beautiful and got along wonderfully when we had our meetings ( walks, brushing, petting, lifting, ect) So we decided to adopt her. On the way home she became a different dog. She was growling at the kids and even nipped at them a few times. We though ok she is probably nervous in the car so they stopped petting her. All this time she was very lovey with me, laying on my lap giving me kisses. Then all of a sudden she turned around and bit my son. he didn't try petting her or get close to her or anything. He went through his pants and broke the skin. So we took her back before we got home. Her foster parent was very understanding, and sorry for what ha happened. I really wanted to get a dane from rescue but now I am nervous. i have found it hard to no if a breeder is a good one or not, with conflicting info telling you that yes a novice can be good, or that you can get a good Dane for a reasonable price.