The minimum sample rate is twice the highest frequency you want to measure.
If you want to detect short spikes, you'd need a very high samplerate.
Get the pulse width of the shortest expected spike, get at least two samples in that duration.
If you want to measure 50 Hz sine, you can theoretically use 100 Hz with a digital low pass.
But, 1 KHz for a few sine periods should work fine for easy measuring. If you can accommodate the memory/processing demand.
When you know your samplerate, you can create an RC filter, (use available tools on the net) and a opamp voltage buffer to your adc.
You cannot sample an CT directly, because the ADC impedance is too low. (most likely less than <50k, and you'd need to charge internal 5 to 10 pf hold buffer each sample)