How to search file content for all users on a business account?

There isn't an efficient way to do this unfortunately, but you can accomplish this by using /2/file/search for each user.

The basic outline would look like this:

• Register an app for the "team member file access" permission and use the "Generate" button to get an access token for your team, if you don't already have one. 
• Call /2/team/members/list[/continue] or /2/team/members/get_info to get each team member's information, specifically their member ID, if you don't already have it.
• For each member, call /2/files/search with Dropbox-API-Select-User containing the member ID, as necessary.

(Alternatively, if the DropboxBusinessAdminTool already does most of what you want, you may just want to fork and modify that directly instead.)