Some old fridges cost way too much in energy to operate and this would be a fraction of the cost to operate. This is the very thing i am working on for my bocks and dark lagers.
I was looking for an estimate on ice change over and two days is fantastic. do you use 1 gallon ice jugs or ½ gallon?
I use both 1 gallon and 1/2 gallon containers and usually dump some ice in in the beginning to get it down to temperature faster.
It's been working well with the current setup. I'm sure there is room for improvement but I haven't noticed a problem with getting a consistent temperature in the water with the water dumping in the top. The water going in is usually 20 degrees cooler too so it will want to go to the bottom anyway since its more dense.