Most of the GB 42s have teak decks. A close inspection should be done to see if they are still bonding properly and watertight. Many of the boats over 10 years old have the teak removed and the decks refinished with non skid fiberglassglass, an expensive job.
The fuel tanks are steel, painted on the outside, on older boats as the teak deck wears down, water gets into the boat around the fuel fill cap on the deck, and then on the top of the fuel tank. For salt water boats this leads to lots of rust, I have not seen enough of these boats from fresh water to give an opinion.
The new diesels will not affect the value of trawlers much if any. Many trawler buyers would prefer simpler technology than the electronic engines being installed today.