How will my existing Apache installation be influenced by the installation of ISPConfig?
The ISPConfig Apche webserver is installed in parallel with your existing Apache installation. The ISPConfig apache is only used for the controlpanel. The hosted webpages are served by the Apache that ships with your linux distribution.
This means you do not have to compile special modules into your Apache in order to work with ISPConfig. You can compile Apache and PHP the way you like.
The ISPConfig installer comments out the PHP settings in your httpd.conf. PHP can be later enabled for each web separately in the ISPConfig web interface.