Would you validate this as a service first or build a small SaaS?

I’m exploring a problem faced by small businesses that receive supplier price lists in different formats, especially PDF, Excel and CSV. Once the data is structured correctly, updating prices is often simple. The difficult part seems to be turning inconsistent supplier files into clean, reviewable data that can be imported into the company’s existing system. My current plan is to avoid building a full product immediately. I would first offer a small paid service: the customer sends one supplier list and a sample of their current catalogue, and receives a checked import file showing the price changes. If two or three businesses pay for this and the same workflow repeats, I would automate only the recurring steps. Would you test it this way, or is there a better validation strategy? What evidence would you require before building the SaaS?