Regarding ethical approval, the university will have an ethics committee and a standard procedure for applying for approval - you usually have to give an outline of your project, copies of information sheets you'd give to your participants, informed consent forms and so on. You may also need to get NHS approval (if this is a UK project) which is normally the case if you're working with vulnerable or young people or need to get sensitive medical data. NHS ethics approval is notoriously slow and involved, so this is something you'd need to talk to your supervisor about early in the process if you start the PhD.
In my experience of PhD interviews they wanted to know more about how I'd approach the topic, what kind of study I'd carry out etc, rather than the stats, but others on here may have had different experiences. I'm a year into my PhD (which is in psychology) and haven't had to commit to my stats approach yet.