First of all please see datasheets and application notes of ICs designated for power measuring. Such IC have a number companies, like Analog Devices, Microchip, Texas Instruments, ... This documents describe circuit around mains as well. Main difference of it's A/D conversion with your task is frequency range of interest, - i.e. up to 1KHz for power measuring instead of 50KHz in your case.
Typically a high ohmic resistive devider is used to apply main voltage to ADC input, and scale factor is adjusted to IC's requirements. Such devider together with antialiasing LP filter gives significant phase error above 1KHz bandwidth. So your task is to chose parameters of this input circuit, suitable to 50KHz bandwidth and accuracy requiments.