Ubuntu's probably a good choice for this application. It's cheap and well supported and reasonably bullet-proof. One way to approach this problem is to set up a "headless" box and do all the administration remotely. Once it is set up, the users could treat it as an appliance and not have to worry about it. I haven't done this myself (I still just tinker with Linux). Some discussion of the various approaches is here: http://ubuntuforums....read.php?t=690895
HowToForge has a nice tutorial on installing Ubuntu Server 7.04 - http://www.howtoforg...t_setup_ubuntu704
The advantage for a non-profit in going with a Linux-based solution rather than Windows Home Server or the like is the freedom from MS's Client Access Licenses and so forth. But a good article on building a $311 WHS box is here: http://www.homeserve...-server_2871.html I have no personal experience with WHS, either.
Hope this helps a little. Good luck!
Cheers,
Scott.