I've had loads of au pairs and have great relationships with them. It's a bit of a mine field at first but can be great. A few things I'd think about are your critically important factors such as level of english or driving etc. I usually go for slightly older au pairs over 24 as have found that works best for us.
I speak lots of times on skype and whats app to try to get to know them better. Really it's about the relationship and honesty between the two of you that makes it work. Different ages of children require very different things. My children are all now at school so i need different help that I needed when we had them at home.
Happy to chat on a pm if you'd prefer - or via email email@example.com