OK, your post was a bit unclear so I will do the best I can. With the cat...if you need to replace it you might as well replace it with a high flow unit and free up the exhaust a little bit. Stock cats can be a bit restrictive. I would give Random Technology a try (www.randomtechnology.com
). You can get a universal cat in any diameter and any number of ins and outs you want. With the gas...what does it say in the owner's manual? Try 91 and see if it knocks. If it does, then go to 92 or 93. If you can't find anywhere what it needs, then you may just have to experiment a little, but generally speaking, Mercedes run on Premium.