I assumed this is a post 86 300e, based on the mileage, since you did not mention the year . The earlier 300e are well known for problems with guides and seals. How much do you drive? What did you mechanic recommend? Where are you? Are you planning to keep the car? etc, etc. It sounds like a valve job ($1500-2000 depending where you are). A GOOD MB independant can do it. Get a warranty.