Proposal a La Scrum

I have a potential client that has a list of features and changes he wants for an existing internal app. He would like me to make a proposal.

However, as: - I haven't seen this internal app - nor have I seen the code - nor do I know if this guy is going to be nit-picky

making a proposal seems futile. A feature can take 4 hours, or 4 days, I don't know.

At my normal job I just work in sprints, which work great as every party involved knows what is expected from each other. So I would like to propose he takes on a couple of sprints from me, and we can discuss together what feature has priority.

Does anyone have experience doing it this way? What would be the best way to approach this?

Thanks