You can use RO water to achieve the lower hardness that you are aiming for. In terms of pH, you can achieve that with CO2 injection.
If you are planning to inject CO2, I would recommend you use a solenoid, as it will help you save gas in the long term.
In general, about 30 ppm of CO2 is a good concentration to have. The amount that you actually are injecting varies depending on size of aquarium, diffusion method, etc.