The main advice I would give about winches is that gear ratio matters. Besides choosing one of adequate capacity (1500#, 2000#, 3000#, etc), make sure it is at least 4:1 gear ratio, and higher is better.
As others have said, a strap (instead of wire) is fine and friendlier to work with.
And I agree with others about the folly of pulling on D-rings, but the way I do it seems to work. I pull from the 2 rear D-rings (on either side of the center stern D-ring), running my stern rope under the boat to the bow, forming a V, then clip the winch strap into that V (you want the V to be a foot or so back under the bow). This V line is best rigged while the boat is floating in shallow water, then float the boat around to the rear of the trailer and clip the winch line into it. This divides the load into the 2 D-rings, and by running the V strap under the boat, it also lifts the bow up onto the trailer. Also, I loop a 4-5' strap through the bow D-ring and around the winch strap, to keep the bow from rising too high as it approaches the tipping point on the trailer roller. With it rigged this way, I can trailer a loaded raft. I don't often do it, but on a crowded ramp, I have and it works fine. And though a helper is nice, I can load it by myself. Another thing I did on my previous raft, was to tie a loop (a butterfly hitch) in each side of the chicken line, right where the rear rocker starts, and I'd clip my V line into those loops (and under the boat), which spread the load around the rear 1/3 of the boat's D-rings. That worked well, too, and obviously, my chicken line was stout enough for that load (a loaded raft).