I don't think there's a way to have it automatically download and install them, without having it also reboot (if no user is logged in) if needed.
Your best/only bet may be to Enable the "Configure Automatic Updates" policy object and set it to "3" so it will automatically download them, but won't install (and therefore not reboot) until you tell it to.
3 = (Default setting) Download the updates automatically and notify when they are ready to be installed
Windows finds updates that apply to your computer and downloads these updates in the background (the user is not notified or interrupted during this process). When the download is complete, the icon appears in the status area, with notification that the updates are ready to be installed. Clicking the icon or message provides the option to select which updates to install.