System design already uses filtration, then chlorine injection to atmospheric tanks for contact time, then booster pumps to distribution. I'm wondering what is the most cost-effective treatment process to add to the system, considering the following options:greensand filter,aeration,water softener,proprietary oxidizing filters,sequestration by polyphosphates.
The greensand filter is probably the best option. If your hardness is above 120, then you should consider adding the water softener in series.
The aeration system is difficult to work in a small system of  Slurry pumps and will not work with low pH. Considering only the information you've provided, the raw water iron/manganese levels you've listed are low enough that I would not recommend the aeration.  I assume these are your only constituents of concern, but as Gary mentioned, you should certainly compare your analysis with primary and secondary standards. 
Having said that, I would continue to use filtration.  However, I would suggest switching the location of your chlorine injection so that it's injected prior to the filters.  Inject enough chlorine at that point so that your 2mg/L is available as residual after the filter.  Chlorine injection will act as a particularly suitable oxidation process at these iron levels and will greatly increase the amount of iron removed in the filter.  For that flow rate, I'd recommend a pressurized filter vessel at least 14 inches in diameter.

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