We spent three winters living aboard in the UK and have so far completed one in Venice where we now live and where the winter is even colder (but shorter). I would agree that a cockpit tent is a big help and we would not want to do without our drip feed oil heating with radiators (a Kabola). A dehumidifier makes life a lot better too. Without it condensation is a major pain. We knew people who kept nice and warm with their diesel cooker which heated their boat as well as cooking their food.
I would suggest investigating the price of marinas in winter. Many have a winter deal which is far more reasonable than the rest of the year. We found that the less popular marinas did deals on their list price and were happy for us to live aboard. In winter we found the ability to walk ashore essential and took every advantage of the showers, laundry etc and I suggest if it is at all possible then it is well worth budgeting for. if it allows you to heat with electricity rather than oil then you should recover some of the extra cost.