Custom CMS for Photography/Virtual Tour Company

I am looking to have a CMS developed which will be hosted on Google Cloud / App Engine. This will be focused on the business of Photography, Virtual Tours and Video Tours. I have a particular design in mind and would like to start with something basic while adding features over time so future proofing is very important. Features to start out will include: - account management - client creation with login (I like Mozilla persona but am open to ideas) - project creation (would like to look a bit like Google Card concept) - item creation - file uploading - file downloading - various email functionality (email download link to client, etc) Features to be added over time will include: - Virtual Tour publishing using KrPano - Video Tour publishing using Vimeo & Youtube as host - automated emailing - Freshbooks integration - Google Analytics integration - project statuses, project updates, etc - the list goes on from here