Children of this age are old enough to handle the truth........as long as it is given tack fully and with concern for her understanding. While she is old enough to hear the truth, she is not old enough to understand details such as the main reason for the divorce. A simple "Mommy and Daddy were not happy living together so we decided to live apart" is all that is needed. Children thrive better on simple answers then more complex answers. If she asks questions be honest.........but keep it simple and the less details the better.
As far as introducing your new "Friend" this is acceptable.......as long as you keep in mind that she is young and may need some time. Keep your relationship easy going in front of her. No intimate behavior for a little while. Let her know this is your friend and you will be spending time with him and his children. She may ask you if this is your boyfreind.....if so be honest. But again keep it simple. Something like "Yes we like each other as boyfriend and girlfriend so we are going to date" Most 7 year olds know what boyfriends and girlfriends are so this is not so shocking to her. If she asks how daddy feels about it........again keep it simple but explain that you and daddy do not like each other as boyfriend and girlfriend anymore but more as friends. And that daddy is happy that you found a new special friend. (Daddy should back this up if he can).
Children are quick to adapt as long as both mom and dad handle this well. The less fighting or arguing the better for your child. In the end she is the most important part of all of this and you both must work together to make this right.