Hi yinliliang,
Based on Microsoft's Windows IoT Enterprise licensing model, the typical approach is for the device manufacturer or system integrator to purchase Windows IoT Enterprise licenses through an authorized Windows IoT distributor, rather than having end customers purchase Windows separately. You can certainly customize the Windows image yourself using tools such as the Windows ADK, DISM, Sysprep, and provisioning packages. However, you'll still need to obtain the Windows IoT Enterprise license through an authorized distributor, who can also assist with OEM activation (e.g., OA3/Digital Product Key) and licensing compliance.
Your proposed workflow: customizing the OS, installing your company's software, deploying it to your hardware, and selling the complete integrated appliance is the standard deployment model for Windows IoT Enterprise.
I'd recommend engaging one of Microsoft's authorized Windows IoT distributors early in the project. They can advise on licensing, activation, image deployment, and manufacturing requirements to ensure the solution complies with Microsoft's OEM licensing policies.