"Classic" Plastic Boats
Most of us have "classic" plastic boats. We benefit from the fact that fiberglass boats can last so long, and retain all of their shape and strength. This makes for a robust used boat market with prices around 10% to 20% of the price for an equivalent new boat. The reason you should use a surveyor when buying a classic boat is that despite the incredible durability of fiberglass, there are a few issues that make a few boats costly to refit.
Water intrusion and the resulting rot or corrosion are is by far the leading issue with all boats of this age. When we bought Lady Lillie, a '77 h27, we could see that a PO had rebedded the lifeline stanchions, due no doubt to water intrusion, and rotting of the wood lamination in the deck under them. The surveyor noted moisture (although no deformation) of the mast step. So the second winter (after two summers of sailing) we saw that the step was deforming around the compression post, and interior frames, so we took her to a good fiberglass shop, and had the mast step rebuilt at a cost of around $1,100.
6 years later, I rebedded two of our portlights which were leaking because of the deterioration of the wood in the cabin walls. We will replace four more portlights this winter. Because I am doing this work myself, and rebedding, rather than replacing the portlights, the cost is less than $100 (one boat unit) per window.
So we will have spent less than $1,500 repairing the effects of water on wood in 11 summers of sailing Lady Lillie. I am confident that you can (with the help of a good surveyor) find a Hunter 31 or similar boat that will serve you as well!
As another note, we have averaged about $1,400 per year for upgrades (a bimini, a canvas shelter, leading lines aft, electronics), repairs and replacements (new sails, new standing rigging, a new engine) on Lady Lillie over 11 years. I would plan around $2,100 for a 31 footer in similar condition, based on the costs increasing as the cube of the length.
Good luck in your search!