Suggestions for client website

Hi all! I have a client who specializes in selling, maintaining, and repairing generators. They want to keep track of customer data (customer details, appointments, schedules, etc) and inventory of generators (stock, on back order, etc. It won't be like a webshop or anything though.

I've created simple websites for my clients in the past, but a request like this is a first for me. Where/how do I even start?

Any help or direction is appreciated!