I have teak
decks on a 23 year old Tayana V42 that I completely refastened and recaulked. I replaced about 15% of the planks that were worn too much at the plank edge to cut a new caulk groove. No screws in the new planks. Its a lot of work.
Some of that work was done part time on weekends and some was done full time after I sold my business. I can't even guess at the total hours or cost involved. A lot of both.
A 20 year old boat will need be be refastend and recaulked if it has not been done already. Take a look at this deck and note the cracks in the caulk and the caulk has pulled away from the edge of the plank.
Tayana 42 for Sale in Florida
That deck is past time to recaulk. If your boat looks like that then you have to do it or pay someone a lot of money
to do it if its worth recaulking at all.
Look at the plank wear around stanchions, deck fills, anything that sits on top of the teak. The planks were probably 1/2" thick on a 20 year old or so boat. Any more than 1/8" of wear probably means refastening and recaulking is a lost
Look for plank edges that are worn so much that the caulk groove is gone. Need to cut a new groove and recaulk if there is enough plank thickness left or replace the plank.
Tap the planks lightly between and across screw holes. Planks that are no longed bonded to the substrate with polysulfide caulk, most likely what was used, will sound hollow. Hollow sound means the plank has separated from the caulk and is held by screws and edge caulk only.
Teak decks in really bad shape will probably feel spongy in places. Look inside lockers that are open to the underside of deck. You might see evidence of leaks
there. Remove overhead ceiling and look for leaks
Caulk that is proud of the deck planks indicates time to recaulk has passed.
Have someone walk around the decks while you listen below. Creaking and cracking sounds most likely indicate plank separation and/or glass delamination
If you decide the decks are OK or salvageable, get a surveyor
that is familiar with Taiwan teak deck
boats and see what they say. Some surveyors don't know squat about teak decks.
I think the bottom line is that unless you are comfortable with refastening, recaulking, and replacing some planks yourself, that's assuming that the deck is salvageable, its gonna cost more that its worth to have someone else do it for you.