Got a Florida import 560 where the seller claims it's an 300hp engine. I have my doubts, since it's a US model. Question is, did the us market get any 560s with the RUF engine? It's an 88 so it can't be ECE

It wouldn't have come out of Sindelfingen for delivery through the MBUSA dealer network with 300hp. Maybe someone with connections... Do you have the VIN to check the datacard? Can you dyno it as part of a PPI?

This thread might help - ECE and RUF 560 - Mercedes-Benz Forum

Sixto
83 300SD
98 E320 wagon

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But I didn't find any info on it regarding the engine, it's nicely equiped though.

I bought it, and it IS a 300hp RUF engine in it. Interesting car indeed, also has a 10way speaker system with amp still in the trunk, never seen it on an Euro before

Take a close look at the car, it might have been a gray market car brought into the US.