Welcome! As a recent buyer and now a late 80's 42 owner - location (east or west coast or elsewhere) matters a lot. I don't believe there is any major construction differences from early glass until around '92? when the 42's at least got around 8-10" larger in beam - which makes a HUGE difference. As I am sure you have discerned from reading these forums - GBs all have a set of 'red flags' - fuel tanks, teak decks, windows - which really, at 35+years, is about an individual boat - and you can't generalize about all boats of that era.
While any individual upgrade doesn't necessarily move the needle on price - the totality of the upgrades over the last decade might matter..
When we looked - there were huge differences between mid 70's and late '80s - not so much within a 5-6 year period. Of course GB started offering much larger engine configurations - depending on how you dream of using your new boat - that might matter.
We did look at 36's - there seem to be more options - only forward cabin, port and starboard doors or not, etc - these might be very important for the admiral!
If you provide some concrete comparisons - I am sure we all will chime in on 'value'!
