As long as it is above 0 degrees Celcius, go ahead! We sail trough the whole winter here as long as it stays above 0 degrees. If it is below, the sheets freeze and the deck is covered in ice.

We use drysuits now, but have used wetsuits earlier. I would not reccomend using a wetsuit tough. If you decide to use a wet suit, put on some polyproylene or wool under it. Put on wool socks inside plastic bags before you stick your feet into any neoprene boots.
Some surfers neoprene gloves with open palms keeps your hands warm, put on a warm hat and you are clear to go.

Good luck!