Re: Please tell me this is a nutty idea so I will stop looking at this boat.
Check that spares are available. The head or block could be cracked. Assuming the engine is original then it is 33 years old and spares may well be in short supply or out of stock totally.
Also my spidy sense tells me that it is not going to be just a head gasket. They are about $30 and it takes 1 - 2 hours to replace assuming nothing else is wrong. A $4k drop for a head gasket.
Hmm plenty boats with running engines out there.