A short description is fine - what's prompting this, and what state things are currently in.

It's fine if this isn't fully worked out yet. Working that out is often part of the job.

Roughly how many photographs, what period they span and where they're currently kept - cloud storage, hard drives, old albums, boxes of prints or a mixture.

A rough figure or range is fine. If you're not sure yet, just say so - it helps me understand the likely scale of the project.