Winters in the Cape are truly a range of no-problem weather - to big-problem weather.
My best advice would be to plan a trip with elasticity in it. You may indeed have a day here and there where you would rather not be out and about. Lodging with a fireplace would be an asset.. just in case you need to stay put one day.
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I would say take a variety of clothes to layer yourself. It is very cold at night, and can be cold and wet during the day, but can also be very warm during the day, from what they call a "berg Wind" with is a hot wind blowing from Central Africa. Make sure that you stay where they have heating, electric blankets, etc. However, its a great time for Game Viewing, long walks along the beach. There are great shops, restaurants and the cinema if the weathers too bad, but there are also lots of lovely bright sunny days. You will love it, just wrap up warm with a fleece.