If you want an old unix workstation you might also consider something from SUN, an ultra 10 or something. Solaris is the most popular form of unix (by sales) and still in very active devlopment - not sure about IRIX. The other advantage of the old sun workstations is they can run the most recent versions of solaris and modern solaris apps, even Solaris 10 when it comes out (january).
Disclaimer: I'm an SE for Sun so I may be a little biased