Most reliable all weather performance comes from keeping things simple like getting a Shimano Nexus Inter 3 or if you have steep hills the Nexus Inter 7 speed hub but this is more expensive of course. These hubs are sealed to keep out everything that can be thrown at it and are maintenance free. You will probably need a chain tensioner if you decide to go this route or jerry rig the current rear derailleur as a tensioner by adjusting the stop on the derailleur to keep the chain on the sprocket then remove the cable.

Then go with disk brakes in the front, even though their not as reliable over the long haul they do work better in mucky weather, but since the Nexus hub uses a coaster brake which is unaffected in mucky weather you may not need to go the expense of doing disk in the front.

Also Lizard Skins sells a neoprene headset seal the goes around the outside of the headset, I use one on my all weather bike and they only cost $4 or so.

Stay with the fenders, they keep the bike cleaner.

If the hub is too expensive all you can do is to use wet lube and clean and relube the chain after every mucky ride.