I have fitted a winch onto a standard tow bar step. It is slightly less capacity than this one and pulls our Ribcraft 585 just fine.
I douse the cable in wd40 after each use.
Also, I didn't want to have a winch that went over the ball. How are you going to attach the trailer once it is up to the car? IMHO the triler brakes can't be relied on! well, at least mine can't.
PM me if you want a picture of what I have done. It certainly works well. We use a very shallow slipway and it saves the car from the salt.