DreamingofExpeditions wrote
"... proven older than 21 years of age for the complete vehicle, no restrictions ..."
Thanks for the link. 21 years old may be the cutoff for EPA, but NHTSA has additional rules about safety equipment that allow non-conforming vehicles to be imported only if they are at least 25 years old. So the net effect is you either pay to have a newer vehicle upgraded to USA safety standards (expensive things like bumpers, side impact bars, airbags and so on), or you wait until the vehicle is 25 years old.
Scroll down toward the bottom of the following document, and you'll see that importers have successfully petitioned NHTSA to allow importation of many models of G-Wagen.
http://www.nhtsa.dot.gov/cars/rules/import/graymarket_RI_list010410.pdf
At least you can find out what changes need to be made to successfully import a 1990s G-wagen, thanks to the work done by Europa International and others who stepped up when Mercedes decided not to import the G-Wagen.
Chip Haven