Integrating users' Dropbox files within my SaaS

Hi all

Long time Dropbox user, but new to Dropbox integration and the possibilities therein.

My company is creating a SaaS platform for catalog and product management. I would like users to be able to share their (selected) Dropbox content with us (in particular images in a specific folder) to enable us to complete various aspects of our workflow.

We would need to copy these images to our (PHP/Laravel) server and then run various scripts/conversions to output the images for our WordPress integration and to create different types of catalog content.

I don't yet understand the best way for us to gain authorization to access a folder within a user/company's Dropbox account and monitor this on a very regular basis. Dealing with file changes (additions, updates, deletes) is a huge part of the catalog workflow process.

I would be very grateful if a suitably experienced user herein could share with me how this should work according to best-practice techniques supported/approved by Dropbox.

I'd prefer that a user doesn't have to reauthenticate every 72 hours or have other stumbling blocks to having a great information management workflow.

Any tips gratefully received :)