What is the roof pitch of the house? What type of metal are you looking at, exposed or concealed fasteners? Would I recommend installing metal over existing shingles, NEVER. The shingles underneath will continue to deteriorate and lift the metal, this can compromise the lap seams and also pull the screws out.
Screw down metal panels are agricultural panels and are not designed for installing over living space. Pigs and cows don't care about getting wet, but you belongings inside your house will. I see people put screw down panels on all the time, will they leak, not a question of if, it's a question of when.
Strip the roof down to the wood decking, install your new metal roof over a material like Weather Watch, Storm Guard or any non granulated peel and stick underlayment. Take the static (box vents) out and use ridge vent.