Sometimes MB actually has the cheapest deal on cats, shop them around - there should be someone cheap in the Atlanta area. They are sold as two cats on a single pipe, you order right side and/or left side as needed. They are typically about $800 a side. By the book, you replace both sides, the O2 sensors, and the air mass meter but you would be well into the $2k territory. I would probably just replace the right side (they are not too bad to replace, be sure to break the bolts loose while the exhaust is hot if you can, use the PB blaster) - shouldn't take more than a couple hours to do. Clear the codes when done, and be suspicious of any rough running afterwards. If you have an air mass causing the mixture to be too rich your new cats won't last long.