Call the dealership as recommended above and price out a re-manufactured unit. If the price is agreeable with your budget, then shop around for a reputable indy who will warranty your Mercedes-Benz part and install it for you.

I am just suggesting a course of action. I think $700 is a very reasonable budget.

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2006 BMW M5 "Heidi" @ 109,000 miles
2005 MBZ C55 AMG "Lorelai" @ 165,000 miles
1991 MBZ 300E "Benzachino II" @ 165,000 miles
1990 MBZ 500SL "Shoshanna" @ 118,000 miles

(On the hunt for a good used M103 engine as of 6/10/23, PM me if you have one to sell!)