Carbon dioxide (CO2) itself ...

Carbon dioxide (CO2) itself does not disinfect water in any way

Carbon dioxide (CO2) itself does not disinfect water in any way, so the water should be treated to control disease, especially if the source is surface water.  Depending on the water chemistry, CO2 will buffer the pH of the water, but really has little, if any, benefit to the water chemistry.   I would be concerned about corrosive conditions that may result from excessive CO2.  A marble lime test is a quick test for corrosivity, but is not a good test overall.  A metal coupon test, using a coupon or coupons of metals the same as those found in the water piping system (including piping in the customer's premises), would be more useful.

I don't think I would be very concerned about CO2, unless it is causing the water to be fizzy, or is interfering with chemical treatment being used to treat the water, or is causing increased demand.  Aeration would probably help in both cases.

More important is an initial comprehensive analysis for such things as the basic chemistry of the source water, toxic levels of metals and naturally-occurring toxins, and organic compounds, both those organics that may be toxic, or may react with disinfectant chemicals (such as chlorine) to form undesirable compounds.  The initial tests, of course, will be important in designing the treatment process, and periodic testing to verify that the treatment processes are accomplishing their intended tasks.

Without knowing more details, I can't offer much more insight.  Once the water chemistry and various other conditions become known, there are many others more qualified to help with the details of designing an adequate treatment system for drinking water.