Map the system
Front-end framework.
Backend services.
Database.
Authentication.
Third-party integrations.
Hosting.
Deployment.
Monitoring and maintenance.
Look for comparable systems
Relevant architecture and integration experience can matter more than a long list of programming languages.
Clarify ownership
Repositories, cloud accounts, domains and production credentials should have clear buyer-controlled ownership.
Full-stack does not mean infinite-stack.A candidate can be broad and still need specialists for security, infrastructure, design or other deep disciplines.
Use inspectable milestones
Break the build into functioning pieces rather than waiting for one giant final delivery.