It’s supposed to be possible to set the default search engine in Windows Internet Explorer using a custom unattend.xml file with sysprep. This technique uses the SearchScopes configuration under Microsoft-Windows-IE-InternetExplorer.
If you’re maintaining images of Windows 8 or Windows Server 2012R2, you may have come across problems where these SearchScopes fail to apply correctly. Sysprep will run successfully, but on startup Bing will still be the default search engine in IE. After some investigation, the issue appears to be related to having certain (now obsolete) Windows Updates applied to the base image.
To resolve, search for KB2976627 and KB2987107 in the installed Windows updates, and remove them if found. Reboot after removal, followed by a full check for new updates. Install any pending updates as normal, and re-run sysprep. Assuming your SearchScopes are properly configured, they should work now.