Any of you from colder parts, when (and if) do you switch over to winter tyres?

It's still mainly over 4 degrees here, but for example this morning it was 0 and icy.
Normally it only snows for a week or two a year and I was thinking of leaving the car at home when it snows, and just not buying winter tyres at all.

Another idea I had was to buy a used set of tyres, since I don't need them for a full season. There's a big choice of used tyres, some even come with metal wheels which would make it easier to swap over (ie I could do it myself). Is there anything to watch for here? For example I've seen a good set of Continentals the same type (195/50-15) but I'm not sure if the wheels fit... are they a standard fit?

Any advice is much appreciated!