How big of a buffering tank did (do?) you use and are there any specific kinds of insulated tank that are better for this? Quick math seems to indicate that a 50gal tank provides about 10K BTU of reserves with a 72*F setpoint. Of course, there will be standby losses, and I'd expect to want/need a decent reserve for the hottest days. A very interesting idea!
Just about any insulated tank should suffice. There is no reason it couldn't be a standard hot water storage tank. The sizing would simply affect the cycle time of the chiller, ideally. Larger tank, longer cycle times. Comfort levels would be unaffected either way, just so long as it is piped correctly. (Ideally, the air handler would always see the ~42°F water). The amount of buffer for a given sized tank is a function of the difference between the chilled water supply and return temps. Larger temp difference, and the more BTU's a given gallon of storage will provide. A taller/skinnier tank will provide a better stratification between the cold supply water at the bottom and the warm return water at the top.