You don't need anything that expensive for 10 to 20 users. Get a cheap white box (anything over 800MHz will do), plenty of RAM and a decent hard drive, and install either Linux, Open Solaris or a varient of BSD on it. Then install Samba for file sharing, Postfix for mail, and any other stuff you might want. Job done.
If you really want to push the boat out, get two and have one as a full back up.
Even with both boxes it's likely to come in cheaper than the two you quoted.