The 2 hours they quote is probably the book time from MB, if you are inclined to do it yourself there is a link somewhere on here to a very informative "HOW TO" with pics that takes you step-by-step through the process.
i replaced my 6 spark plugs in about 20 mins. if you are doing it just ensure you screw them in by hand to begin with to avoid cross threading & if possible, use a torque wrench at the end.
the 20 mins i took didnt take into account cleaning the spark plug area etc before removal. my engine (M110) is a nightmare for this but yours may not be as bad. IMO well worth doing along with cleaning of the HT leads at the same time.
definatly a job you can do yourself anyway - 6 sparkplugs (BOSCHE) from the main dealer for my R107 was £12 from the dealer! The plugs are probably more expensive for your model and you need more (i think) but still worth doing the labour yourself, if you are happy to get stuck in
With Turo, you can book and drive any Mercedes you want, wherever you want it, from local hosts across the UK, US, Canada, and Germany. Test drive cars you otherwise might never get a chance to try, or hire something interesting for your next holiday: www.turo.com