Hi pamw,
I don't have a huge amount of experience in these things, but maybe you could ask the PI some of the questions you've asked here? Eg it would be fine to ask them if there's a project already specified, or how much scope there is for the post-doc to develop the project themselves. It's pretty much always seen as a good thing in academia to have your own research ideas and interests, regardless of how well defined the project is, so I'd probably think about what kind of things I would like to do, that would fit along the lines of what's in the ad description. Then in the email you could briefly mention your areas of expertise and how your interests fit with what is described, and ask whether you would be a suitable candidate. Also, if the ad is vague that's a really good sign that they don't already have someone in mind for the job :)
I think that's what I would do, but maybe others on here with more experience can also advise ;)
Good luck!