|Two-thirds may sound aggressive, but this really depends on the starting point. What's the going rate for similar craft in your area? Ditto for areas within reach - major metropolises aren't that far away and there are a lot of "committed" (= desperate) sellers out there. Just as a point of reference, a 27 that sounds like the mirror of the boat you are considering (Mk III, lightly used, never upgraded) sold quickly at my Toronto club for $15k asking (we don't know the final price).
As with the boat you are looking at, much of the inventory was tired to the point of uselessness. Thirty+ years doesn't matter much for anchors and rodes, but at that age, the fibres in the sails are beginning to break down to the point that they damage easily and aren't repairable; you could end up with a split mainsail in the middle of your much-anticipated cruise. By the same token, the foam in the cushions is probably turning to powder.
I suggest that you have the boat surveyed. Lightly used is often coupled with "lightly looked after" and as we all know, the smooth sheen of good gelcoat can hide a multitude of issues. You will need the survey for insurance purposes if you buy, and a good surveyor will be able to give you some indication if the price is reasonable, or a reasonable justification for offering less.
C&C 27 Mk IV