Winches should be sized based on the sail area being handled. My owners manual shows a short, optional track at the front edge of the coaming that I presume would be for bringing the existing 110 back to cockpit winches or for a larger genoa (not advised) or possibly for a spinnaker. I fly an asymmetric and route the sheets to blocks on the stern rail and forward to the cabin winches. I would certainly prefer to have cockpit winches but for my use, the added expense is not worth it. I can't tell where your main sheet is installed (some 280's had it at the pedestal); so it is possible with winches on the coaming, you could control things from behind the wheel. If I were going to add winches for an asymmetrical spinnaker there, I'd use 16's.
I tried some searches a few years ago for decent used self-tailers but didn't have any luck. Try Googling and see what you can find.