You are now getting somewhere!
Your system is an FFT processor. Your sample rate choice is correct but you also need to consider an anti-aliasing filter in front of your ADC to limit the input signal bandwidth (unless you are sure that there is no signal above 300kHz). A higher sample rate simplifies your filter.
The next step is the resolution you want to achieve with yor FFT. Im most applications, 8 bits is enough but it really depends on your final goal.
Try to use a system model using matlab, for instance, to understand how the resolution of the ADC affects the FFT output.