That 57 Fisherman is in beautiful cosmetic condition for sure. The question is whether or not it’s mechanical condition is that nice also. Do a close inspection, checking compression, looking at the gear lube condition (will probably be dark brown/black which is OK). Have a close look for damaged/cracked castings that may have resulted from being dropped or mishandled. You will need to assume the engine needs an ignition and fuel system overhaul, as well as a new water pump impeller, unless the owner can show it was properly serviced somewhat recently. The final question is whether or not the engine includes a decent (not rusted) pressure tank.
My last question is how much do you want this particular hp/year engine? You would be hard pressed to find another one in such nice cosmetic condition. If this engine is a “must have”, then you should be willing to pay a little more for it. Does the owner have an asking price? If so, what is that price? Don’t insult the seller with a low ball offer, that might just turn him off to selling you the engine at all. You can use the fact that the engine will need a good amount of servicing before it can be used regularly. The lack of a pressure tank (or if its tank is internally fouled/rusted) is a great negotiating point also….
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