Do not use the Bosch platinum plugs. They have a tendency to break! FOr more specific info, run a search. Go with the regular BOSCH copper plugs set to OEM specs (gap).
As for the coolant, you can only buy it (to my knowledge...) (in the US anyway) at Mercedes' dealerships, and it's MERCEDES coolant, simply. So you won't find it at your local auto parts stores. I believe I payed about $11 for the bottle.
Also, the differential fluid should be changed every 30k miles (or at a 'B' service, aka 30k mile service). And the coolant should be changed every 2 years, I'd change it along with the other fluids.
CONGRATULATIONS ON THE CAR THOUGH!

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2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)