For the roads that you are more than likely to encounter, I'd suggest going to a local salvage yard and pick up some 17" wheels off any Mercedes. (But if they have an SLK, that would be ideal). Then buy a 50 series tire to get more sidewall thickness and that will give you a little more ground clearance. 17" tires usually cost less than the size that came on the XF, so it might be enough to counter the price of picking up some used wheels. Your car is going to look a little funny, but since you're going to be using it more like an SUV, utility out weighs style. I think forum member "FP" used this set up for winter driving, you could get in touch with him and ask what size tires and wheels he used. Good luck.