Guidance On Product From Scratch

I am currently working on expanding my skills and resume and am currently out of my wheelhouse. Background is I have typically been developing in house software and SaaS integrations/migrations(B2B); more heavily in the product ownership role.

I was approached about creating a small piece of custom scheduling software for a friend of a friend and am struggling how to start and proceed to see if it feasible. Things need to figure out are cost, scope and timeline or if there is even a solution out there already(not sure how to go about research here there other than googling).

I have started an initial interview with the client to understand basic needs, problems they are currently facing and budget. I am not sure what else I am missing or what kind of documents I should start creating or questions to ask. For actual development are there already created foundational software bundles we could develop on top of?

Any insight, guidance or ideas would be super helpful. Thank you! (Additional catch is it may need to be HIPAA)