James,
The filter is not needed if you have an oscilloscope to measure the channels. You will need the filter if you use a DMM to take the measurements. I don't know exactly how steep the slopes need to be, but 12 dB should be ok as a start. If you search the web there are filter calculators to help with the component values. If you go this route just find a bread board and put the filter together that way. Use the cheapest components you can find.