We suggested ebay to at least find a disc for your 15 year old software because you already have a key that you know is legit.
We DO NOT recommend you buying a different Server OS/License off of eBay.
If it is a new OS on eBay for cheaper then retail then odds are it is a stolen key, which means as a business running windows server you can face very very large fines for using that stolen key.
So in windows server world things have changed over the last 15 years:
CAL is a user/device license. You get to pick one or the other so if you have 15 computers and 6 users you need 6 user CALs, if you have shared terminals and say 6 computers/devices and 15 users then you need 6 device CALS. Now interestingly enough microsoft does not require you to input the CAL license into the OS, merly hang onto it for record keeping in case of an "audit".
Also, you need a completely seperate version/CAL if you want the users to have remote desktop capability to said server.
Another change is the cost of OS for virutlazation or Servers with more then 2 physical CPUs on the baord.