[v8volvo]

Gates belts are fine. Continental, Flennor, or Dayco are all also fine. These should be available from any seller you check with, either online or from a local store. Makes no difference where you buy them, so just use a seller you already trust.

Timing belts unfortunately do not give reliable signs of needing replacement in terms of visible or functional symptoms. Age and mileage are the only good indicators, which is why knowing when the belt was last done is critical. Other than those, the only indication you will get that the belt is at the end of its life is the moment that it breaks and damages the engine.

This is also why you replace the belt, even if it seems like it might be fairly new, anytime you do not have concrete, documented knowledge of the last time it was replaced, or if there is any doubt as to whether the job was done correctly using the correct tools. You will see the same recommendation for any other interference design with a timing belt too, from Toyotas to Subarus to newer gas Volvos and virtually every other modern engine with a timing belt. Same reason with all of them.

You said you got all the greenbooks, which is good. Did you read through the timing belt replacement procedure and note the special tools that are described? Can you ask the owner you bought those books from where they had their belt service done or if they have the tools?