If you plan to launch in salt water, I would strongly recommend a galvanized trailer and not chopping up one you already have. Just pointing this out, you can not safely weld or grind a galvanized trailer without a fresh air system.
I consider a winch a requirement. Plus front and rear tie down point for towing on the road.
Some reading, with pics. I wish my trailer had side bunks, and someday I might add them.
Inflatable Boat Trailers: Using Small Boat Trailers