FM Pro14 is the desktop version, not the server version. Yes the desktop version can host a solution for a maximum of 5 clients, however the FMS14 (Server) is a completely different animal and you would need to give more information regarding number of concurrent users, number of files and file sizes before a considered recommendation could be given.
Filemaker's own suggestions are outlined here http://help.filemaker.com/app/answers/detail/a_id/14401/~/system-requirements-for-filemaker-server-14
By in large, FMS14 requires processor cores, more concurrent users requires more cores. A fast and robust disk subsystem and memory(even more if web direct is being used). However that being said, it can run quite comfortably on lower spec hardware if the load is light. As for OS Platform for a server, I have 2 opinions on this.
1 ... the common sense approach. There are far more cost effective hardware solutions with inbuilt redundancy available for the Windows platform. This may well be a deciding factor for those who are looking for a robust enterprise class server solution to run FMS14 on.
2 ... the "I use Mac, OS X and iOS for everything because it just works". I am firmly in this camp with a "good backup and fall over strategy (just in case)"