I think step 5 lacks

cd /var/www/
chown -R -v -f www-data:www-data typo3_src-4.2.1/*

 in order to have the right permissions for several TYPO3-installations in ISPCONFIG.

Creating several TYPO3-sites I had problems with the symlink in step 7. As if the symlink from former installation interferres with next installation. I get 403 not permittet  when trying to start TYPO3 install script (site.org/index.php)
My solution was to unlink symlink for the next installation and link it again.
My install script ended up with:

cd /var/www/webxx/web/
wget http://surfnet.dl.sourceforge.net/sourceforge/typo3/dummy-4.2.1.tar.gz
tar xvfz dummy-4.2.1.tar.gz
rm *.tar.gz
cd dummy-4.2.1
mv * ../
cd ../
rmdir dummy-4.2.1
cd /var/www/webxx/
chown -R -v -f webxx_webadmin:webxx web/*
cd /var/www/webxx/web
chgrp -R www-data fileadmin typo3conf typo3temp uploads
chmod -R g+w,o-rwx fileadmin typo3conf typo3temp uploads
cd /var/www/webxx/web
mv index.html old_index.html
touch /var/www/webxx/web/typo3conf/ENABLE_INSTALL_TOOL
cd /var/www/webxx/web
unlink /var/www/webxx/web/typo3_src
ln -s /var/www/typo3_src-4.2.1 /var/www/webxx/web/typo3_src

Somewhere on the internet I found that the following had to be pasted into Apaceh Directives in ISPCONFIG for the site:

<Directory /var/www/web5/web>
AddType application/x-httpd-php .php .php3 .php4 .php5
php_admin_flag safe_mode Off
</Directory>