It depends on how you want the car to look. I bought my Msport with 17" standard M sport rims. Then knowing that i wanted to get rid of run flats and get a certain 18" rims for the car simply made my 17" my winters. IMO better look then steel and hub caps.
You could get a set of 17" alloys M sport or other and stick on them plenty on ebay and just order the wintery tyres from eventtyres.co.uk
Knowing dealer prices will probably work out similar as the dealers steel wheels
Titanium Silver E90 LCI 320D M Sport Saloon
- Logic 7/ USB Connectity and Bluetooth/ Visibility Pack (Xenons, Adaptive Lights,Auto Wipers Lights and Corner Lights) / Sun Protection Glass/M Spoiler / Brake Ducts / BMW Performance Pack / BBS Wheels / SS Cat Back straight through with BCS Backbox