Just to add on to what the others have already eloquently stated...I'd be prepared to completely rewire the boat as well. A lot can happen over 40 years and who knows how many owners who think they know a thing or two about a thing or two. Random wires spliced into random circuits should be expected.
If this is your first boat, and you haven't already plunked down a deposit, I'd be looking elsewhere. Boat show season is fast approaching so you'll find some owners looking to offload their current boats so they can upgrade. You might stumble into a gem of a boat.