All three variants of Honda City gives similar mileage.... its mileage in city is around 13kmpl while on highway it gives 17kmpl.... out of three its one variant Honda City S AT comes with automatic transmission option... all three variants are equipped with same engine... they differs only in terms of luxuries.... for more detail about this car you may visit the above link... Honda City
Certainly, you can get your car modified as per your choice. Coming to the front grill, you can replace it with the different grill. For that, we suggest you to get in touch with the authorized service centre as they will be the best people to suggest you few compatible grills to choose from.
Click here to find the Honda service stations by selceting your city in the drop down list.