Hmmm...a 1970 steel boat. Probable value = displacement in tons x spot price of steel/ton!
Unless the boat has been kept incredibly well for 35 years..chances are that it is worth substantially less than FG boats of the same age and level of finish. If you like a particular boat I would offer substantially less than asking price...maybe 2/3 and then haggle and make your final offer contingent on survey. Surveyor can then tell you how much more the owner should take off to alllow for the defects he will find.