^I agree pretty much with all of what @peakbagger said but feel that the self adhering Ice and Water Shield is overkill given the quality of now available synthetic underlayments (e.g. Grace Tri-Flex). Don't even consider using tar paper fro underlayment.
As with most things, the devil is in the details with things like how you stack vents are handled. In general you want to rely upon gravity for sealing rather than sealants.
Edit: the roof I'm currently replacing with metal is a 50-year fiberglass shingle roof that started leaking a and falling apart at about 30 years. Composition shingle warranties are probably a good indicator of value relative to other composition shingles but I wouldn't bet on the roof lasting out the warranty period. OTOH, metal roofs have well demonstrated their longevity.
Edit: federal tax credits may also apply
I’m surprised to see so many recommendations for raised seam steel. Yes, mine may last 80 years, but it will look so ratty before half of that time has passed, that I will replace it..
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