There is a great deal of information about Stirling engines here.
http://en.wikipedia.org/wiki/Sterling_engine
By definition, Stirling engines use only gases for working fluids, and then only gases that remain in the gaseous state throughout the entire cycle. Helium is a good choice. Good old air is not too bad. Stirlings are not turbines, and unless you are burning hydrogen as a fuel, Stirling engines do not produce water.
Kamen's Stirling engine drives a generator (or more likely alternator), and because it is a Stirling engine, it does so efficiently. This can be a very efficient way to produce electricity from non-conventional or renewable fuels.
The Kamen invention (Kamen did not invent Stirling engines or alternators) is the "Slingshot" water purifier. Apparently, it is heat driven and it's claim to fame is that is very efficient,it recovering most of what would otherwise be waste heat. There didn't seem to be any claim that waste heat from the distillation process is used by the Stirling engine, so the two units could be looked at as semi-related. The Stirling generator makes electrical power from almost anything that burns. The Slingshot can use that electrical power efficiently to make potable water.
If you've got a Stirling generator, then the Slingshot might be a good addition, but if all you want is potable water, then a still might be the way to go. Yeah, you've got to boil the water, but apparently that's what the Slingshot does. I would wager that a maxi-efficient still could be devised, driven directly by burning cow chips, needing no Stirling generator, and costing much less.
(Sorry Bob! You posted just after I checked for new posts)