You need to first choose a linux distro that matches your needs, and then see what it already provides. If you are new to linux, I'd suggest something package based such as Fedora (fedora.redhat.com) or centos (
www.centos.org)
Firewall: iptables (you might want to look at a front end that will ease the setup)
Also, make sure your linux server is the gateway for all the machines on the network, ie, it has sole access to the external Internet connection.
Mail: postfix
Comes standard with most distributions, but not necessarily enabled by default. On Fedora and Centos, you need to either change your preferred mail using "alternatives", or simply uninstall sendmail. (rpm -e sendmail)
Ftp: vsftpd (Very Secure File Transfer Protocol Daemon)
Also comes standard with FC and Centos and has an entry in /etc/init.d to start and stop it. (have a look at "man chkconfig" to enable it upon startup.)
Web: apache
rpm -qi httpd will give you information as to the package, and rpm -ql httpd will show you where it resides on the filesystem. An /etc/init.d/httpd start will kick it into action on
http://[ip of your server]/
The default setup is very secure, but read up on the stuff you add to it (php scripts, perl cgi's etc) to make sure they contain no vulnabilities.
I'm afraid my knowledge of radius is limited, but
http://www.freeradius.org/faq/ should answer enough questions to get you started...