yes, yes, and no. The no is regarding the need for a registered domain. There are several free dynamic domain services like
http://www.dyndns.org/ that will allow you to keep your host name updated for whatever your IP address is. You will have to choose a host name on one of their domains though.
However, sharing ISOs is not going to work very well unless you have a T1 or higher. Even with cable/DSL the "upload" or "upstream" or "send" bandwidth is usually very low which means very long transfer times if people are downloading ISOs from your machine. Going the other direction (to your machine) would be very fast from a server residing on a T1 or higher network.
And sure there are many ways to configure Samba. You can configure it as a Windows Domain Controller, a Domain Member server, a Workgroup server, etc. But of course then you are just learning *Windows* networking. Still better to do it with Samba. If you want to learn UNIX networking then you'll want to play with UNIX services like NFS, NIS, dhcp, bootp, dns, telnet, ftp, tftp, ssh, etc, etc, etc...
