How powerful of a pump do you need in GPH?I loathe prechillers. The cost of copper is so much more than a suitable pond pump to pump the icewater directly and it's nowhere near as efficient.
Something like 200-300 GPH would do it so you're looking at a $20 pond pump:
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=47117