Mounting new, to me, winches...

Backing plate and sealant depends somewhat on whether the winch can be serviced from the top or not. Does it need to be removed from the deck to service the bottom bits? Butyl tape it the way to go for most everything. A properly made boat may not need a backing plate if the layup is designed for a winch. Stainless is too heavy for the stiffness in my opinion, but cruisers here love a low riding slow boat.......