In my opinion, the most corrupt, criminal, dishonest and unscrupulous people in the building trades are foundation repair guys. Roofers are a close second. Of the dozens of roofing companies in my area, there is only one that I trust. I've done dozens and dozens of jobs fixing what they did wrong, and that's only the jobs that I accepted, some are so bad that the only option was to tear it off and do it again.
Mueller is a metal building company that manufactures and sells all their own products. They are very good. When I talked to them about putting metal over shingles and their warranty, I was told that there had to be an air gap between the metal and the shingles. Wood purlins are a must. 1x4's would be the minimum, but for a better finish, 2x4's are better.
If I was hiring a roofer to install metal to my roof, I would insist on a complete tear out of all roofing materials, flashing, paper, and vents. Then I would want a complete inspection of the decking. It's truly amazing how many houses have shingles over huge voids in their roof that are nailed to just the felt paper. Then I would want brand new paper, or even better, a synthetic underpayment and then install the metal over that with brand new flashing and vents.