I would like to set up one of my spare computers to be:
* an intranet web server for Mediawiki and Joomla and
* a file server to store all files from 5 other computers and
* attached to an all in one printer for all to share
Overly ambitious? Well, no, not necessarily. Piece of cake? Well, yeah, pretty much... depending.
So, in a nutshell, Apache (web server), NFS (file server), CUPS (print server), router(s) (your ethernet connection between machines). And, of course, MySQL and PHP (particularly for Joolma, which I know nothing about but Wikipedia is so very helpful). You don't need to be an expert for any of those, but you will have to do some reading, fiddling, swearing, more reading, more fiddling... step by step, one at a time. If you throw Samba into the mix, you get that PC supported (more reading, fiddling, cussing...).
Which box? One that's reliable, has, oh, a gig or more memory wouldn't hurt, and a good-sized second disk drive, too. That 900MHz box might not be bad or whatever else you've got that's not a three-legged dog with a busted tail when it's running. If you've got ethernet cards in those printers, that makes life simpler, but it's doable with USB connections, too. You only need one LAMP box, the server, the rest of them only need a browser to talk to it.
I'm doing pretty much this with Slackware (on all machines I have), a Linksys router, H-P printer (ethernet), DSL to the outside world... Oh, yeah, you don't have to have wired ethernet, a Linksys wi-fi works fine, too.
Read, fiddle, cuss, read, fiddle...
Best of luck with it.