I have worn my O'Neill Psycho II 3/2 through December depending on how cold the water is. I will tell you that my upper body will get very cold, but the unbearable part is in the shins, where you have 2mm on a part of your body that has little fat on it. You will def need a thicker 4/3mm+ suit come Jan, if not well sooner.
Has anyone ever tried only wearing a 3/2 in the winter? I dont want to buy a thicker.
I did this and trust me, it sucks. You'll get really cold, and your hips/legs will go numb, definately not worth it.
Layers just make it worse, and you feel real heavy and have no flexibility.
My advice is to hit all the shops till you get someone to cut you a deal. Thats what I did and came out with a sweet winter suit for 150$ cash, brand new. 350$ suit new. Usually someone will have a 4/3 or 5/4/3 that they want to get rid off, so just walk in and make them an offer. Good gear makes all the difference, bite the bullet and spend the money.