Karl,
I think your wife will definitely appreciate the lack of clutter under the sink and the great efforts you will go through to deliver RO water to the kitchen.
There really is no magic to a DI tank. My Agie has a simple stainless cannister with a filter screen on the inlet and a very fine mesh filter on the outlet. Remove the lid, dump in half a cubic foot or so of resin, close it back up and hook up the inlet and outlet line. Works exactly as you say; when the conductivity gets above a certain level, water starts going through the resin tank.
If this is your first wire edm machine, be aware that it's generally a good idea to allow the filters to do the work of cleaning the water
before water is allowed through the DI tank (especially with aluminum). i.e. Get the conductivity as low as possible before doing your cutting, then adjust the "threshold" number up so that there will be no water going through the DI system while cutting, then after the water gets cleaned up fairly well the threshold value can be set down low again to run the water through the DI.
Your resin will last much longer doing it this way.
This is more important to do when filters are new, as new filters simply don't do as good a job as ones that have been used a bit.
Obviously there are times when this is impractical, but for short jobs, this is the method I generally use.
PM