I have a 2 post hoist that can be used in asymmetrical (doors open to clear the post) or symmetrical mode and have used it to store 2 cars in one bay when not servicing a vehicle. My floor was newly poured in anticipation of installing the lift with 15cm (6 inches) thick high strength concrete. Being a 2HP electric hydraulic lift, it requires a 230 VAC 1 Phase 25 amp circuit. I agree with Colin that if you only have one lift, it should be a 2 post since I perform all of my own mechanical work. A four post is a great option for minor service and easy vehicle storage.
One thing I have to stress is do not buy the cheapest lift available. Contrary to what many people believe, you will never get out of the way of a falling vehicle. In Canada and the US, vehicle lifts used in a professional shop must be ALI Certified (American Lift Institution) that insure lifts that are covered by this certification are tested to 3 times their rated capacity and have all the latest safety locks in place. I made sure the hoist I purchased was new and ALI certified since buying an old lift from a repair shop was not an option since this to me is false economy. It is your life, do not penny pinch on safety.