Welcome aboard, and good luck in your search. The GB 42 is a beautiful and capable boat.
There are several well populated threads on the Forum that may help you.
Boat search 101, the Grand Banks section under makes, and individual threads about buying, using, or selling GB 42s. The Library icon is at the top of the main page.
When we were looking, we found that Yachtbroker.com had the nicer boats, at higher prices. Boats.com and boattrader.com had a wider range of prices, on boats which ranged from very nice to salvage / junk. Craigslist in major boating cities may also be a place to look.
You already know that Grand Banks changed their hulls from wood to fiberglass in 1974, for most models. Many of the cheaper boats are wood.
If you have sorted out how and where you will use her, you may be able to find a good boat that the PO has already added the bells and whistles you may want for your intended use.
It might also pay to look at other boats in that size and quality range, to see other arrangements of similar spaces. Good luck and good hunting.
JohnS, Cheoy Lee 40 LRC