At what point is a boat worthless?

If you feel the 60 is a good boat and well maintained I´d go for the 60. Better start with a boat in good condition and make it yours by enjoying it than start with a porject and spend a fortune on a refit without getting the pleasure of cruising for another 2 years (that will cost money to keep the boat somewhere as well).
Keep the offer reasonably low as I would expect the owner is more interested in handing the boat over to someone who really takes care of it than getting big money. The engines are not state of the art, but certainly strong and sturdy beasts. Why care if they take 20% more fuel? More modern engines will burn fuel as well and cruising is never for free. And you probably won´t burn the diffence in fuel consumption too soon. And if the boat is in good condition and you don´t have to invest big money in making it look good again you certainly will loose less money once you want to sell it again.

A yacht is worthless when a boat yard has to cut it up to throw it away, because the last owner abandoned it, and they can't find anyone to buy it at any price.