Who will actually do the work?
Ask to meet the people writing your code, not just the people selling the project. Seniority on the call should match seniority on the build.
What happens after launch?
Find out how support, hosting, updates and small changes are handled, and what they cost. A great launch with no plan for afterwards is a risk.
Who owns the code?
You should own your code, content and data outright, with access to the repository from day one. Anything less makes it hard to ever change supplier.