Should we be prepared for interviews at this time? → If you arrange early enough, you may be able to get 1~2 interview with schools that you won’t be able to visit later for an interview.
Should she have basic information readily available? → I believe it will make a difference if she knows enough about a school to ask a genuine and meaningful question specific to the school and herself. The school won’t be asking her academic states at the fair.
Can we just window shop? → Absolutely. However, there are not many schools and not many attendance. And there won’t be too many applicants for each school. At least not too many to prevent those AO’s to remember you and your daughter anyway. It’s better to be ready to give a better first impression even if you only plan to window shop.
Is it worth a 6 hour one-way drive to go or are there better options? → It’s worth. And it will be more worth if you are prepared and share ride. Better yet, if you had already visited and interviewed at a school, you can use the fair as an opportunity to connect with the AO.
I have seen overdressed parents and kids but I don’t think it had any negative impact on their admission. Well dressed people are always eye pleasing. I would wear business suit if I were to do it again. For your daughter, read other threads like this;
http://talk.qa.collegeconfidential.com/prep-school-chances/1398507-what-to-wear-for-the-interview.html
Or go to the school’s website and social media to see what style their students are wearing.