My personal opinion on the matter - on a car like the M3 just leave the suspension stock. Its already low enough from the factory imo where it doesnt look like you are driving a 4x4.
Also, nothing will ride as comfortably as the oem suspension. Ive lowered a few of my cars in the past and its always exciting at first and you think it feel much improved. But the ride quality decreases gradually over time. Its not until you swap the suspension back to stock or ride in an equivalent car that you realize "damn this is way more comfortable".