I recently replaced the rear muffler assembly on my 1986 4240 SEL. I am pretty sure it is identical to the muffler you will need for your 500SEL. The cheapest price I was quoted for OE was $450. That was from Caliber Motors in California, a Benz dealer that doesn't mark up from SRP and discounts to MBCA members. On average the quotes were $600. The original muffler on my car lasted 120,000 miles and 10 years. The replacement was an ANSA and it lasted 50,000 miles and 4 four years. As for midas I don't know. I actually put a Walker Mufler on mine. Saved only about $100 on the deal, but I got a lifetime warranty (as long as I own the car) I plan on owning the car for a long time and MB only offers a 1 year warranty. I really wanted to go for the OE, but the warranty I got was too good to pass up. If the thing dies every three years (probably will) I get a new one free. They only take about 15 minutes to change. I do think that the OE is worth it, and unless you are planning on owning your 500SEL more than 8 more years, I'm sure it will last.