It looks like they have to be less than 25 years old.
http://www.nhtsa.dot.gov/cars/rules/import/ELIG071404.html

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1970 280S M130 engine- good runner
1971 250 M130 engine- #2 rod bearing, gone
1971 280SE (blown engine,parts car)
1977 German 280S W116-only 33 years old

25 or older are exempt. Under 25 years are subject to NHTSA import restrictions. It would be a good idea to see if the same applies to EPA regulations. Also check out the recent posting on the Forum about potential problems with insuring grey-market cars in the US.

Happy Motoring, Mark

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DrDKW

OK, a person can get an exemption if the car is not on the list (W108 is too old to make the list):
a person may not import into the United States a motor vehicle manufactured after the date that an applicable Federal motor vehicle safety standard (FMVSS) takes effect unless the vehicle complies with the standard and is so certified by its original manufacturer. This prohibition applies to both new and used motor vehicles, but does not apply to motor vehicles that are at least 25 years old. (See list below)

The process probably requires a person to jump througha lot of hoops :

Under one of the exceptions to this prohibition, found in 49 U.S.C. § 30141, a motor vehicle that was not originally manufactured to comply with all applicable FMVSS can be lawfully imported into the United States,
provided it is determined eligible for importation by NHTSA, and is imported by a registered importer, or by a person who has a contract with a registered importer to bring the vehicle into compliance with all applicable FMVSS following importation.

__________________
1970 280S M130 engine- good runner
1971 250 M130 engine- #2 rod bearing, gone
1971 280SE (blown engine,parts car)
1977 German 280S W116-only 33 years old