I have a 2006 56 model BMW 520 d SE Auto which I plan to keep for two winters. They are now on 225/50 run flats on 17 inch alloys.

1. Do I just fit same size and profile winter tyres on the same alloys? My local tyre shop has same spec run flat winter tyres, part worn, 4.5mm thread and asking £360 for four including fitting
2.Is it worth getting new steel rims of smaller size, if so which? Is the expense justified given I am keeping car only till spring 2013?
3. Do I need to purchase run flats or could normal tyres suffice? ( I have pressurised tyre sealant with me)