Imo substrate is more important than CO2. I have plenty of plants growing well in 3 tanks with low light, no CO2 (but max allowed fish), and iron-rich substrate. Florite or Eco-complete are great choices for planted tanks. My newest tank has black flourite sand.
You can probably get away with gravel or something for certain types of plants that don't require special attention as long as you supply root tabs and other ferts.
And then of course there's a good number of plants that don't need substrate at all...various mosses, java fern, watersprite, anarachis, etc.