Here is the US the X5M comes standard with 20". I think he is not looking to buy new rims. I am running the 275/40/20 Pirelli Ice and Snows all around on the stock 20". While the tire place did complain the it was difficult, they got it on and it works great.

I didn't go with the 315 because TireRack did have them in stock when I went to buy them.