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All good advice, but if it is a Mercury, and turns and looks good, I'd offer $400 (the crank alone is worth that), and might go as high as $600 if it looks to have never been stored outside.
You might get lucky. I bought an unknown '51 Merc about 25 years ago for $400, and it ran so well that all I did was clean it up, re-gasket it, and paint it. I'm still running it. Other than excessive oil consumption when running at 65+, it still runs good and strong.