Sampling Frequency and Bandwidth
So you want to measure the mean value and not the waveform. At first place you have to determine what are the capabilities of your MCU. You will neeed an analog low pass filter on input of ADC to elliminate aliasing effect, then you can oversample and do a digital low pass filter.
For example: 1M sampling, 330kHz RC low pass filter, then a digital low pass filter - slighty higher cutoff frequency than the entire execution loop, let's say 20kHz.

BW of opamp is not an issue, you will hardly find an opamp with BW less than several Mhz. The high BW is desiderable an not viceversa, you do filtering with passive components RC, choosing a low BW product with the aim to eliminate high frequencies is a wrong way.