Typically if it has a US format VIN it's not a gray market car. The VIN will be something like WDCCA24xxxx. CA24 is a 300SE. It'll be something different for a 380SEC. Gray market cars usually have VINs like WDB126043xxxx. You can also run the VIN by any MB dealer and they'll tell you if it's a US spec car.
In evaluating the what you should pay for the car, check what a running car up to your standards would cost and deduct from that amount what rebuilding this SEC will cost.