Don't pay any attention to the folks who are telling you to buy a different radio unless it fits your use case.
I could not agree more. The little FT 817 / 818's are great little radios.
As for the CW filter......if money is an issue, you may get by with an external audio filter? I say, "MAY GET BY" because it depends on band conditions and how you use the radio. If you are not operating in crowded band conditions, or maybe an afternoon setting up portable in the park with an antenna that is not great, you may not be bothered by other CW stations.
But, if there is a CW station that if very close to you and you can see your S meter bouncing around because his signal is so strong, an external audio filter is not going to clean that up. His signal is strong enough to keep on interfering with the signal you are trying to copy.
There's nothing like a good CW mechanical filter designed to be use with this radio.
My advise is, "save your lunch money and get the CW filter designed to work with your radio. "
You could get the external audio filter and get by. Down the road you may say to yourself that it's time to get the proper filter. Now you are out the bucks you paid for the audio filter. If you get the CW filter first, down the road you are not out any bucks.
Barry, KU3X