You want to buy your software through the Microsoft Licensing program, not through any retail outlet or online at the MS site. You can get the best deals when you buy all of your licenses together - server, CALs, Office apps if you're using them, etc. When you buy through the licensing program you get only one CD and one product key (actually, MS charges you about $25 for this). You cannot, however, buy through the licensing program except through a reseller. Some of the on-line resellers, like CDW for example, will sell you licenses, I believe, or you can go through a local reseller.
Since you have under 50 workstations, you should consider Small Business Server 2K, which includes Win2K server as well as Exchange 2K and SQL 2000,as well as modem and fax sharing server software. This is a really great deal if you want the whole MS scenario including email, office apps, etc.
As far as documentation, again I'd have to refer you to the MS knowledgebase - support.microsoft.com. Search for white papers, etc., here.
Basic steps would be:
1. Make sure you have both a PDC and BDC in place in your NT 4 domain.
2. Decide whether you are going to (a) upgrade on your existing systems or (b) replace your hardware.
3. If (a), upgrade your PDC first (required), then the BDC(s) and then the workstations.
4. If (b), create a new domain/forest by installing Win2K fresh on your new hardware and do a migration from the NT4 domain to the Win2K domain.
Hope this helps!