Can softwareupdate be used to automatically download (but not install) on the terminal?
From man softwaredownload:

-d | --download
    Each update specified by args is downloaded but not installed. The values of args are
    the same as for the --install command. Updates downloaded with --download can be sub-
    sequently installed with --install, or through the App Store (as long as they remain
    applicable to your system).  Updates are downloaded to /Library/Updates, but are not
    designed to be installed by double-clicking the packages in that directory: always
    use --install or the App Store to actually perform the install.


So yes, that is possible. AFAIK iCal has lost the ability to run AppleScript commands a few releases back, so you might be better of with a launchd job.