For some general and fairly comprehensive information about the 500SEC in general, take a look at
www.mbcoupes.com
If you are in the USA, and are on cordial terms with your dealer's parts department or service advisor, you could request that they run you a copy of the MB VMI "Vehicle Master Inquiry". This is at their discretion though, and don't be surprised if they decline to do it. The MB VMI will disclose the date of manufacture, port of importation, dealer where first sold, options on the car when new, and warranty claims made on the car. The VMI has codes, mostly in 3 digits, for the names of the dealers, equipment on the cars, and codes for the nature of the repairs performed. Unfortunately, there won't be the names or contact of prior owners on the car.
Also, you can write to the Mercedes Historical Archives in Stuttgart, and send them a copy of the title, and with a cover letter requesting a copy in English of the factory build sheet for the car. It takes about a month to get a response, but they will give you one.
Good luck sorting out your car. The worst place for rust on the upper body of these cars is underneath the rear window, when water leaks in there, also they sometimes leak into the trunk through the tail light seals.
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1991 560 SEC AMG, 199k <---- 300 hp 10:1 ECE euro HV ...
1995 E 420, 170k "The Red Plum" (sold)
2015 BMW 535i xdrive awd
Stage 1 DINAN, 6k, <----364 hp
1967 Mercury Cougar, 49k
2013 Jaguar XF, 20k <----340 hp Supercharged, All Wheel Drive

(sold)