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!