I don't really know what does that Server App actually provide for OSx already provides many sharing features.
http://www.apple.com/osx/server/features/
The only reason I'm considering the Fusion drive is because my only Mac is now a MacBook Air 2011 and running stuff and the OS itself off its SSD drive is a whole different game!!! I just got rid of our late 2011 MacBook Pro base model because I couldn't stand the beachball. It popped up too often.
A *server* is a whole different game! You won't be booting it often (my last bootup was 35 days ago) or launching programs. And if it takes an extra 0.01 seconds before you video starts showing, because you have a HD rather than an SSD, you won't notice it. You also won't notice higher end processors or GPUs. The only thing that will matter is the drive capacity, if you don't have enough. Put your money into drives.
Note that all bets are off if you are using server as a processing farm (such as with Final Cut), a web server with many simultaneous clients, or to run large databases. But home users don't do these things.