The manual mentions installing multiple packages in one syntax regarding the mail part, which is perfectly fine:
"aptitude install postfix postfix-mysql postfix-doc mysql-client mysql-server courier-authdaemon courier-authlib-mysql courier-pop courier-pop-ssl courier-imap courier-imap-ssl libsasl2-2 libsasl2-modules libsasl2-modules-sql sasl2-bin libpam-mysql openssl getmail4 rkhunter binutils maildrop "
However I wasn't paying close enough attention to the output/feedback of the scripts of the various packages and missed the message:
"Postfix is not set up, start with
cp /usr/share/postfix/main.cf.debian /etc/postfix/main.cf <snip>"
This caused the following error message when setting loose the ISPConfig3 install script, and stuff didn't work afterwards:
PHP Warning: copy(/etc/postfix/main.cf): failed to open stream: No such file or directory in /root/temp/ispconfig3_install/install/lib/installer_base.lib.php on line 789
I copied main.cf as described above and started postfix, without issues.
So thanks to till for the above and below, then I reinstalled the postfix packages:
apt-get install --reinstall postfix postfix-mysql postfix-doc
And then ran the ispconfig3 update script instead of install script, and chose the 'reconfigure packages option':
php -q update.php
Admin page reachable and able to login after that, so all working fine.
I have the new ispconfig3 manual, so stuff is only starting now.
Thanks for your help!