Some snow tires make a big difference, and turns the Z4 into a pretty decent winter sports car, if you ask me...you'd be wise to invest in some if you plan to be driving in snow frequently
DSC off would probably not be good with all the slippery roads, but I think DTC turned on is what you want - lets the wheels spin more than normal, but the other safety nets are still there...just press the DTC/DSC button briefly until "DTC" lights up in yellow in the tach