im not going to be a HUGE techincal help but i hope to give you some food for though.
1. distro really does not matter. pick one you are comfortable with like RHES, or WhiteBox or one of those forks, or any other distro like slack, debian, gentoo, etc..
all distros can do the same stuff, the issue is finding one that works best for you.
2. you are asking a lot to have security, FTP (major security issues there), web server, plus LAN services... you might want to consider multiple servers for this for security issues.
have 1 server that is for your FTP and e-mail and web server. that alows you to put them out on the DMZ if you want to help protect the LAN data and work stations. then put up the 2nd server inside the firewall for everything else. i have a little whitebox server running apache (web server) and qmail (e-mail server) that is protected with clamAV connecting to 4 LAN workstations and about a dozen WAN workstations.
all that server does is e-mail and web with web-mail service that only a few end users touch. this way if something happens to that box it will not comprimise the rest of the LAN and the data stored on the LAN side server.
samba and cups can handle your login, file permissions, and print sharing with little issues. you might run into some issues (search around here for win2k3 AD and samba) if you have any win2k3 AD servers running. win2k3 and samba as your kerberos do not play nice along with LDAP. you will want both kerberos and LDAP running on the linux server for the windows boxes for the security and to emulate the AD they will be looking for.
hope that helps a little bit.