More often than not you won't need to buy any parts to keep your bike working well. But your battery is questionable at this point, so it may be time to have it load tested and replace it if it isn't fit anymore. Many auto parts/retail battery stores will test it for you free. You didn't mention whether you have any aftermarket lighting or other possible electric power hogs wired up, so I assume your bike is stock except for the winch?
Beyond that minor bump there is preventive maintenance to keep up with. Start by unplugging and checking every plug in the harness to verify that you have clean, dry, corrosion free connections. Put some Dielectric grease in each when you plug them back together so they'll remain clean and watertight. Clean the battery terminals and cable ends along with the starter & ground cables going to the motor and frame. Take the shift motor out and clean out the old grease in there too. Inspect the reduction gears for chips or cracks etc. and reinstall with some good quality low-temp rated grease coating the parts.
It also may be time to take your winch apart and clean out the old grease in the sungear sets and housing. Regrease those parts with a quality low-temp rated grease during reassembly. The winch will consume less power and last longer if it is properly maintained, same needs as any other components on your bike.
If your shifting problems still exist after you get through changing the oil & filter & fluids and all the other normal maintenance items discussed here and in your owners manual, come on back and talk about it some more with us before spending money on parts you may not need... and won't fix the bike.
You can find a copy of the service manual for your Rancher
HERE. You'll need to refer to it often to repair and properly maintain your bike.
Welcome to the forums!
