I had the same idea wanting to save on the wheels, looking for a cheaper alternative or a used BMW OEM set of wheels. In the end, I went ahead and spent the money and bought new 18" BMW OEM wheels, new sensors and Pirelli Scorpions Ice and Snow. Total bill? Around $2,500 installed. How much could I have saved? I don't know - but knowing that I'll have the set for a few winters the extra money was worth it to me.
My 2¢ advice is: if you can afford it, spend the money and avoid problems you might encounter buying used or lesser quality wheels.