Almost every job we do has metal roofing on it.
The devil is in the details. Installation matters, tightness of fasteners, thickness of metal, flashing details, seam tape, overhangs, sub-flashings, eave drip pieces, base sheets, framing supports, blockouts, etc. There are very few people who do it well, including many people in the business of installing metal roofs.
Most jobs mess it up somewhere, and rely on sealants to fix problems. Not good.
The biggest challenge is penetrations. Everything that penetrates a metal roof compromises it's integrity. Plumbing vents, make-up air, chimneys, wall flashings, parapets, etc.
I have a current job that was installed by professionals. There are 9 penetrations, and 9 leaks.
I love metal roofs, but I have to be EXTREMELY careful with who does installs, and how the details are handled.
When completed well, they offer great service.
A basic roof is not hard, IF you do a LOT of careful reading of installation methods and techniques. I wouldn't recommend a cut up roof for a beginner.
I wouldn't put metal over shingles. Take them off, and install new base sheet (especially Fire-N-Ice in cold regions to help prevent ice dams).
BTW, cheap metal is usually thin metal with poor paint finish quality. It dents, fades, wears, and fails to seal well. Also, ALL roof screws have "rubber" washers, but the expensive ones are neoprene. They look the same, but the cheaper ones will deteriorate in the sun and leak.