We had our roof replaced here in NJ, it was 2 layers thick and it was about 40 YO (estimate that the top layer was around 20 YO). We replaced it with 30 year asphalt shingles. There are 2 types of shingles that I am aware of totally flat and ones that have texture to them. If you get the totally flat, they are usually cheaper and you may be able to get another layer on top of them if the code allows it these days. We had to replace the top half of our house due to a wind storm coming through and ripping some of the shingles off the new roof being about 6 YO.
You want them to remove all the shingles, examine the wood underneath the shingles and replace any that may be bad. Here in the NE, they need to put a ice shield membrane under the first course followed by roofing paper. You should have all new flashing around skylights and any protrusions coming from the roof. They cut a ridge vent into the roof and put a plastic ridge vent cap along with shingles. Depending on the size of your roof they can probably do a tear off and replace in one day.
Yes you'll find stuff around even though they tarp and try to be as clean as they can. If you can use a strong magnet to pick up the nails, they do sell a rolling device for this, not sure how expensive it is.
Here's a photo of the roof with texture, I think they call it something like Architectural Shingles. I left the photo larger so you can kind of see they have texture to them and the ridge vent can be seen.
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