I would open it up and look at it. Nothing tells me it shouldn't be pretty straightforward. The only prob I can potentially see is the back cylinders might be difficult to reach. Whenever the dealer gives me a ridiculous price I always ask why, you never know if the shop manual stipulates the engine being pulled to access the plugs. Even then 1800 is insane for a "tune-up"

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1981 300TD
2005 C55
2004 E500
1999 ML430
1992 400E
1998 C43
2001 SL500
1988 300SEL
1986 190E 16V
1991 190E 2.6