When it comes to sharing a single IP, I love IPNetRouter. It's a shareware package available widely (I prefer www.macdownload.com). Rather generous terms, and very easy to setup. Best of all, you can try it for 30 days. It also has the capacity to become a software firewall, and lots of other stuff I haven't explored yet. Look into it! (but be warned, some ISP's look askance at sharing 1 IP).
On turning a computer into a file server, just designate a folder as shared, transfer that shared to every folder w/in, the use the chooser on the other computers to connect via AppleTalk to that computer. Since that is the only shared folder, that's what they'll see on their desktop. (I run 2 iMac's connected to an 8500/132 w/ an 8gigHD as my file server/print server, and firewall/router). Works great.