I see that ISPConfig seems to install it's own clamav and spamassassin. I also see email sanitizer mentioned in the FAQ. I'd rather use the packages of clamav and spamassassin that are available for my platform than to overwrite them with yours. And I don't want to use email sanitizer. Can I avoid installation of these during the initial ISPConfig setup? For instance by editing them out from the installation script? (I haven't looked at it yet.) Is there any other software that ISPConfig forces on the machine (with the exception of the software that is installed for the admin server)? In general I don't think that software that is widely available in packages for all major distributions should be force-compiled on a machine with an administration panel. That's how some other control panels do it and that's why I don't use them. Why do you think this is necessary? I see you just released a beta version of ISPConfig 3, do you continue this practice with that version? If so, I'd find that really sad.
You can not avoid the installation of the ispconfig pacakges. ISPConfig does not overwrite any package of the linux distribution. These packages are not availabe by all supported linux distributions with the needed compile options. Also the packages in ISPConfig are newer then the packages in most linux distributions and its always a good idea to use the latest antivirus scanner and not a old one were just securiity issues were fixed. ISPConfig 3 uses only system packages. But it might be nescessary to offer custom repositorys for some distributions later that do not offer up to date packages for older releases.
