Ecomodder? Yikes, that is pretty much a performance terrorist.
Buying a performance car to make it less of a performance car and more of a regular car sounds like a great effort and wasting money and resources. A Prius, Leaf, Volt, city bus, rickshaw, bicycle, moped, roller blades, walking shoes would have all been better transportation options.
Honestly, starting with a Fiesta ST is terrible. Wide sticky tires, bigger brakes, lots of frontal area, drivetrain designed for performance only, draggy body cladding, basically everything a normal person buys a performance car for is the polar opposite of what a performance terrorist would want. A Fiesta SFE with Powershift would be better. Hell, Ford actually made that model for that exact purpose! Trying to ecomod a ST makes as much sense as making a lowrider Raptor.