Believe it or not, I think that is not a bad price for a dealer. I've heard number from $400-500 for the M113 spark plug change.

No, it is not really any different than changing the spark plugs on any other engine. The main difference, is that there are 12 of them and the plugs are typically $10 each or more.

Yes, it is necessary to change them now if they have not been changed. While the mileage interval is every 100k miles, Mercedes also recommends not going more than 5 years without changing them due to the fact that they may start to become difficult to get out of the head if they sit in place for much longer than 5 years. If it's a 99, then it has been 7 years, I would change them.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record