Livia...Here's my thoughts...I'm sure others will chime in and I would also encourage you to read the tons of similar posts here on line and all the answers.
How much does hull strength degrade with hull age? How much of the strength can be determined in a survey?
Very Little and a good surveyor will find any significant issues.
How much of a no-no are teak decks? We've seen articles on removing them - is this possible to do and should one plan to do it?
They are not a no-no...what counts is their condition and evidence of leaks. It is possible to remove and replace teak decks but this is a huge, big, ugly job. You want good teak decks or no teak decks!
If a boat has had blister repair work (see Valiant) is this a permanent solution or would you expect to have to have it repaired again?
A well done blister repair job on the entire hull bottom/rudder should be permanent without recurrence. If someone tells you they've had the blisters repaired...be sure you ask for evidence of a professional fix. If they have...you can feel confident providing the survey shows no underlying problem.
Are wooden masts a definite no? How much would replacing one likely be?
Unless you are buying a wooden boat, I would avoid wooden masts...just too much can go wrong and they are few and far between these days. Besides... do you really want all that extra maintenance?
Is it normal for a broker to request an escrow deposit (and of what amount) while looking for boats for you?
No it is not normal or required. Walk away if he presses this issue. It is normal to make a refundable deposit with an OFFER on a boat...usually 10% of the offer.
What are normal insurance costs? We heard 1% of the purchase price but haven't been able to verify that. Anyone have good experiences with an insurance company and/or know of an inexpensive one?
1% for double handed bluewater cruising is not realistic. I would estimate 3K for a $100k boat. CHECK with your prospective insurance company that they will cover the vessel you are buying and that YOUR skills meet their requiremments for such coverage. I heartily recommend IMIS Jackline insurance as they did an outstanding job for me on a major claim and I know that many SSCA members have had similar experiences with them.
Finally...I think the Tayana 37 is an excellent cruising boat but a little cramped. The Valiants are probably out of your price range in decent shape and I don't think the Tartan is suitable at all to YOUR purposes, while nevertheless being a nice boat. Good luck!