There are as many opinions about wood vs plastic in GB's as there are owners of GB's. This topic is right up there with the best anchor to use and what is the best varnish to use on a GB.
I bought a woodie in 2005 which was built in 1970. It has twin Lehman 120's which had about 6,000 hours in 2005 and now have 8,000 hours. It is my first wood boat.
Maintenance costs are variable to say the least. If you stay on top of the small maintenance problems you will drastically reduce the larger problems. I do not think my "maintenance" costs are any higher than a plastic boat. I do spend more time walking the decks of my boat looking closely into corners for breaks in the paint coverage. I touch up anything that looks like it may let water into the hull or cabin as that is the greatest source of trouble in a woodie.
Mechanically the 1970 GB 42 is identical to a 1974 GB 42 with the same engines. The only difference is the hull, cabin and decks. My teak decks were almost an inch thick and are still almost 3/4 inch thick compared to a much thinner deck on the plastic version.
The biggest cost difference in my opinion is the cost of a covered slip. In the PNW a covered slip for a woodie is a necessity. Fresh water is not good for a wooden boat. Unfortunately a covered slip is more expensive than an open slip.
I had my boat hull and cabin professionally painted when I bought Brassbounder. Other than that I have done all the maintenance and touch up painting by myself although I had new electronics professionally installed.
My recommendation would be to get a very good survey and then consider buying the boat based on the condition and price, not wood or plastic.