Both can be done on the trailer..
but, few things to consider. Is the pin worn and or the hole the pin goes into out of round. If you can get under the boat,and crawl around on the trailer, you should be able to at least feel the hole or see it. If it looks way out of round.Figure on a new pin. But then, it would be worth it, if you buy the boat, to just replace it anyways. The cable is really not that big a deal. Don't try to replace it in the water..bad bad... Remember, the keel is like well over 500 lbs. It's not unmanagable..just need to keep your hands and feets out from under it. there are some good archives on the subject here. I did remove my keel with the boat on the trailer, others raised the boat up to do so. either way, warrants caution.. enuff said.. good luck. Hope you find urself a new boat there.