Ian is correct: using the top K Condorcet winners is a very poor
multiwinner election system, if the goal is to have proportional
representation.
Having studied this a bit, I would suggest that the best currently
available multiwinner election system for our purposes here, i.e., for
proportional representation, is Reweighted Range Voting, see
http://www.rangevoting.org/RRV.html
for details. Advantages: with RRV it is really simple to cast a valid
ballot, ballots can be quite expressive, it is easy to explain how to
vote, and it is quite simple to calculate the winners. The main
disadvantage (cannot do per-district tallying) is not a problem for us.
--Barak.