Win2K Server is expensive. And you'll really want Win2K Advanced Server anyhow - that is really the "base model."
As an education tool you don't even need a new computer.
Shop around for an old one! I have run classroom networks just fine using PII 233 machines with 64MB of RAM and 6GB to 8GB of hard drive space, under NT4 Workstation.
These machines are running IIS (Gopher, Web, FTP, Email), SQL Server 6.5, a third-party Telnet server (freebie), MTS, file and print sharing, and Microsoft NetMeeting in "remote desktop sharing" mode for remote server administration.
For training and demo purposes you generally need more power in your clients than your servers.
I am currently upgrading these classroom LANs to newer technology - mostly due to changes in IIS over the years and budding interest in .Net issues.
To keep costs down I am replacing the PII 233 machines by reworked former desktop client machines.
These "new" servers will be PIIs and PIIIs in the 350Mhz to 533Mhz range. They will have 256MB of RAM and 30GB to 40GB HDs and 10/100 NICs. I will be running XP Pro as the "server" operating system.
To keep costs down I am using MSDE instead of SQL Server (MSDE is a user-limited version of SQL Server 7 that is "free" for licensees of Visual Studio).
Services supported by these new servers include IIS (again: web, Email, FTP), the XP Telnet server, MSDE (basically SQL Server 7), COM+, MSMQ, file/print sharing, MSDN Library, and XP's Remote Desktop Sharing for remote administration.
My hardware costs for these "refurb" machines runs about $250 for commercial-grade units (Dell Optiplex desktops) plus RAM & HD upgrades, and a DVD-ROM drive (software bloat - .Net betas come on DVDs). My software costs run about $150 per machine (refurb machines come with 98SE, I buy WinXP Pro Upgrade at a steal price, and I already have Visual Studio 6 Pro licenses).
The two XP "servers" I have deployed as a test are 350Mhz machines with 196MB of RAM and 20MB HDs. They are overkill for 4-student use!
I will go with more RAM and Disk for expansion and because they are cheap now.
Good luck!
P.S. - For advanced classes I rely on Windows 2000 Advanced Server. There are issues that can't be covered using a client OS - but surprisingly, not that many!