Prestone SuperFlush, or Super Radiator Cleaner, should be adequate. MB specifies a relatively complex citric acid flush. The Zerex stuff won't be at Wal-Mart because they only carry Prestone products. Kragen or AutoZone should have the G-05, or just buy it from your local MB dealer - should be about $12/gallon, which isn't bad once every 3 years...
