I would be much more tempted to buy a Mac mini as a dedicated server, you can hide it in a corner where it's less likely to be put to sleep/rebooted/stolen or messed up over time through normal use. Also they're available with OSX server from day one rather than having to buy the OS on its own. They do a dual disk version now too.
As for how hard they are to set up, well for file and print services they are ridiculously simple, the email and VPN services are simple as well, if you have a decent enough grasp of DNS. I've not personally setup any chat service so can't comment.
This can be done by editing the file /etc/authorization, which controls who's allowed to do what in the GUI in OS X. It's an XML property list file, so you can edit it with either a text editor or Apple's Property List Editor (part of the developer tools). Warning: if you get the edit wrong, it may render the system effectively unusable; test this on Mac you wouldn't mind wiping and reinstalling if necessary. Anyway, you should find a section (under the "rights" main section) that looks like this:
<key>system.preferences</key>
<dict>
<key>allow-root</key>
<true/>
<key>class</key>
<string>user</string>
<key>comment</key>
<string>Checked by the Admin framework when making changes to certain System Preferences.</string>
<key>group</key>
<string>admin</string>
<key>shared</key>
<true/>
</dict>
You can change the group from admin to whatever you want. For example, you could create a group called semiadmin, add all regular users to it, and then edit the authorization file to list:
<key>group</key>
<string>semiadmin</string>
Note that this won't apply to all system preferences. The Accessibility, Accounts, Parental Controls, and Security panes each have their own entries; if want to expand access to those preferences, edit those sections similarly. Also, each computer follows its own authorization file, so you'll need to install this modified file on each client computer (after fully testing it, of course).
Best Answer
You can have a look at http://osxportableapps.sourceforge.net/p_ichat/copy.html. Even if that doesn't work for you, that page has some info on where iChat stores its jabber, and other, settings so you can copy those files to a network location.