If your car was brought in by an individual (Mark Doerfler), then you'd have to find him to find the car's history. Going to
http://www.whitepages.com/person and typing in "Mark Doerfler", I get eleven hits. If you can run the VIN through Carfax and see where it was registered early in its life, you might be able to match the location to one of the names. You could always call and ask anyway.
My car was imported by Richard Buxbaum of Classic Motors in Chicago. Buxbaum was big on bringing in Ferraris, Lambos, Maseratis, etc and when AMG started doing thier thing in the early 80s, Buxbaum secured the exclusive rights to be importer for AMG and created the company AMG of North America. My car was second AMG SL brought into the US - the first one didn't have ABS brakes, and the car was refused by the original owner. I bought the car from the original owner who bought it from Buxbaum in 1983.
The 500SL came with that spoiler, but the Euro versions had the same suspension as the US versions, so your car should not sit lower than a 380SL. The stock 500SL would pump about 235hp, and the AMG version with the polished/ported heads/intake, hotter cam, and new exhaust pumped out just short of 300.