If you have installed Debian base system and nothing else, than you have a minimal set of services and software installed. There is not much to remove that has impact on resources.
What i usually do is to install rcconf and remove unwanted services like exim, portmap, nfs etc... Same goes for that software if you are not using it.
I would remove wget and all other tools that can retrive information to your server.
Rest is hardening the server, because Perfect Server Install is pretty OK install but it is not hardened and that is the job of sysadmin and not ISPconfig or the tutorial. ISPConfig script is pretty safe, it has been so far