If it's NT4, then you should try to put Service Pack 6a on it. If it's W2k, then you should try to put Service Pack 2 on it. Yes, there are different SPs per OS. And that's if you want to keep the OS.
Of course there are different Service Packs. They're different operating systems.
Setting dial-up networking on NT4 and W2k is a reasonably unique proposition.
On W2K it's a snap (all network connections are treated in a consistent manner on W2K). On NT4 it's only slightly more confusing.
But what I think might derail them is that you seem to want to do four or five things all with one NT server: Internet (including firewalling), web server and file server. With only 4 client PCs, I suspect you will get away with it, but NT4 wasn't that good at doing such diverse things all at once. If it gets crashy whilst trying (you should know in a week) then you're asking too much of it.
Nah, there's no problem with this. It'll work.
And creating an Emergency Recovery Diskette for NT4 isn't hard, though I can't remember the command to do it. Unfortunately, it has to put it entirely on one diskette (I don't know why, but can guess). I don't know if it can put it on a ZIP disk. This is different from a Ghost image, BTW.
The command is RDISK and it doesn't have to store everything on a floppy. What it does is store the registry files on the hard disk and the floppy knows where they are.
Ghosting NT4 installations is a bad idea and W2K even more so.
Personally, I think Windows is the wrong OS here; Linux would be a better choice. However, the Linux approach would be more complex to implement (iptables, Apache, and Samba would be a lot of config to swallow, and PPP on Linux can be a bit fearsome for the novice) but hey, it'd work faster and be cheaper. It all depends what you've got more of - time, or money.