Steel wheels and winter tyres.

I have a V8 S6 Avant, 1996, on original speedline alloys (225x50x16 tyres, 16x7.5 ET37).

I am wondering what size the steel winter wheels should be for this car?

I have seen some 16x7.0 ET42 Audi wheels, which may be off an A8 or V8?

What should my car have, and might these be OK?

Are the same wheel nuts used to hold on the alloy and steel wheels, or do I need special nuts for steel wheels?

I know nothing about winter wheels & tyres. My main concern is complying with Swiss insurance regulations, and safely getting through the winter. Any ideas please?