Let the roofers damage the old stuff. And if we get a torrential downpour in the middle of the job, at least it's the old fascia/soffit getting wet. Then either the roofers replace it or another handyman does.
Certainteed architectural shingles actually come in a few different materials. How long each one lasts depends on what they made it out of. A typical asphalt shingle roof, properly installed, lasts about 15-20 years. They can make asphalt shingles that last longer. Other materials last even longer.
When they keep replying "lifetime", ask them what the lifetime of the shingle is - there is no "forever" roof. The longest lasting roofing materials are certain kinds of metal roofs, which last 100 years. So ask them how long in the future you can make them come out and give you a new roof for free.
As for pricing, your best control there is to get bids from at least 3 contractors. That will give you an idea if anyone is particularly overpriced.
WARNING: DO NOT AUTOMATICALLY TAKE THE LOWEST PRICE.
1) If one of them is dramatically lower than the others, that's probably a bad sign for the lowball. For example, two come in at $10k-ish and one comes in at $7k-ish (30% lower). He may be cutting corners, or will tear off the old shingles and find "problems" during the job that require more money until you're paying the same or more than the higher estimates. There are rare cases where someone is doing the work cheap for a good reason, such as being new to the area and thus needing a client history. If he doesn't have a good reason to be way under market price, run. If he says "I'm just better/faster/cheaper than those guys" run faster.
2) You're trying to ensure the costs are reasonable, not the cheapest possible. If you like everything about a particular contractor, but he's a little more expensive, you should go with him. You will be happier in the end.
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