Id use Debian Linux with Samba, Apache and Webmin. Just set up port forwarding on your router so Apache can access the web. As for the machine, im running a webserver with a pentium 3 500MHz and 256MB of RAM and a 50GB HDD. You'd probably want a p4 as the AMDs run very hot (the server was running one) and a much larger hdd. You can pick up a 500GB 7,200rpm hdd off amazon for about £50 (although it has been said to be unreliable)
Password apache (guide here
) for use over the internet and use Samba for sharing files over your home network.