If you want to use stock Mercedes springs you can use springs off of a 2.3 or 2.6 model. You can easily find them used at any salvage yard. This would be the cheapest method. You will probably just need to buy different sized spring pads than what are presently on your car to adjust the ride height. Like previously mentioned, you can also buy lowering springs but they cost a lot more and if you don't want the car lowered then that extra money spent is a waste.