One clarification, there is no Server Advanced in 13. It's just one product. Minor, but just so there is no confusion if others are looking or plain old Server.
Check your hardward configuration carefully if you have that many users on WebDirect. http://www.filemaker.com/products/filemaker-server/server-13-specifications.html
FMI says to use a 2 machine configuration once you get over 6 WebDirect users. And it needs to be a fairly beefy machine to handle it.
As for the OS...see this article. Tip # 1...this applies, unless someone can correct me otherwise, to Windows 8.1 Professional also.