Use NOTHING but the OEM recommended plugs. MB engines can be picky about their plugs. People often change their plugs with "better" ones, and spend tons of time and energy trying to figure out why their car runs like crap.
The reason the dealer charges so much is that the M112 has 12 plugs and several of them are tough to get at. If that's parts and labour, it's expensive, but not out of line.
Besides, what is $300 per 110,000 miles? $0.0027 per mile.
1998 C230 "Black Betty"