The teak decks on GBs are good quality to start with but like all teak decks, it's all about how they were cared for. If a previous owner tried to keep them "teak colored" all he will have accomplished is shortening the life of the teak.
The integrity of the deck seams is absolutely critical. That is where the bulk of the moisture can get under the planks. Once there, it can migrate down alongside the plank screws into the plywood core of the subdeck. American Marine/GB used a really good quality of wood in the subdeck core, but wood is wood and if moisture has been getting down there for some time, a GB subdeck is every bit as susceptible to rot as any other brand.
Our deck is about 40 years old now. It's not in the world's best condition, previous owners over-sanded it and some of the planks have developed crevicing which is usually traceable to scrubbing it with the grain instead of across it or power washing. We had the main deck regrooved and reseamed a few years after buying the boat, but while there are some seams today that need redoing for the most part it's held up surprisingly well.
Removing the teak deck from a GB lowers its resale value unless you find a buyer who REALLY hates teak decks. Most GB buyers and owners feel the teak deck is a big part of the boat's character. I have seen GBs with fiberglass replacement decks not sell until the buyer dropped the price by the cost of replacing the teak deck. For a 36' boat this can cost some $30,000 these days.
If you have questions about GBs I strongly suggest you join the GB owners forum.
Grand Banks Owner's Resources There are people on that forum including some shipwrights and former yard owners with a vast wealth of experience and knowledge about these boats.
I will tell you this now having owned one for over 14 years. GBs are extremely well built boats, but they take a fair amount of maintenance to keep them that way if you want to retain their value. Up until the early 90s, GBs have a rainforest of teak trim on them and this can require a fair amount of upkeep, particularly in this wet climate. The decks are not maintenance-intense (unless they've been let go by previous owners) but they do require periodic cleaning, and they need to be cleaned the right way. And any seams that are starting to pull away from one side or the other of the groove, or plugs that go missing, should be dealt with immediately. However the deck needs to be bone dry when you do this, not an easy condition to have in our climate, unless you put the boat in a boathouse.
The multi-part windows have a number of ways they can leak. The upside is that they are easy to overhaul once you master the technique.
But the bottom line is that GBs are not a boat you can park and forget until the next time you take it out. Unless you keep it in a boathouse. We "screw around" with ours on a year round basis but because I work full time it's all we can to do keep up with the boat, let alone make progress.
In short, they are not inexpensive boats to own both in ongoing cost and in time, either yours or someone else's. Doing all the work yourself on decks, windows, brightwork, etc. can help keep the costs down of course, which is what we try to do, plus we simply enjoy working on the boat. But if you have to hire anything out, it ain't cheap because of the labor involved. For example in the mid-2000s the figure quoted to overhaul a leaking window by yards and the shipwrights in our marina who do this kind of work was a flat $1,000 per window. I have no idea what it is today. We do all our own windows--- which is actually kind of fun--- so we have not followed the hire-it-out costs of this job.
But join the forum and ask your questions there. You'll get good answers, particularly from Bob Lowe who used to own Oak Harbor Boatworks, a yard on Whidbey Island that specialized (and still does I think) in everything Grand Banks, both wood and glass.