anyways automatically updating is a bad idea as sometimes config files get rewritten/moved, and sometimes due to dependency problems, some package gets dropped for no reason. by automatically doing the update, you'll end up wondering why package X is broken or why package Y is missing, and having to check the logs to see where the problem showed up.
IMHO, updating should really be done by hand by a human.
but, hey, you're free to experiment what you like on your computer
if you want to run a cron job with your user, I recommend not putting the script in /etc/cron.whatever as all scripts within those directories get executed automatically by the root cron jobs. put the script in something like ~/cron.daily and with your user, add a cron job with "crontab -e" that points to this script.