Building a Blender cluster with Xserves
If you read the system requirements for Blender, it's pretty wide open. You could run Windows XP or newer, Mac OS X version 10.5 or newer and of course you could run Linux or perhaps even FreeBSD.

So, it's really your call which OS is easiest for you to maintain. For free (or pretty near to free) you could get a used copy of Snow Leopard or Leopard for probably $15 or even call Apple or go to an Apple store with the serial numbers and see what OS was shipped with those Macs. You might have to pay a shipping fee, but they will mail you replacement DVD for the OS.

You certainly don't need server for this software (reading the wiki linked in the question - it's clear the software handles all the clustering needed as long as all the devices are on the same network and can talk).

It depends on how expensive your time and what OS you like best to run and install. Mac is clearly the easiest since all the drivers you need will be vetted and ready to go. Once you have the software installed and running - should you find in discussions with the blender community that another OS handles a specific task better - you could always jump ship to that OS later.

Looking over the Xserve specs, you might not be able to run 10.5 but unless you have a big reason to not upgrade - I'd run the latest client version of Mac OS that will run on that hardware.