What kind of underlayment, etc should I request the roofer put between the metal roof material and plywood?
High temp rated peel and stick membrane.
There are several to choose from.
I used deck armor and storm guard. I believe most people don’t go the extra mile for deck armor and use tiger paw which is readily available. Both went down easy. We had 50-60 mph winds the next day and it all stayed on to my surprise
Old school is felt or ice shield, then a layer of rosin paper to act as a slip sheet. The more modern approach is high temp ice shield and breathable synthetic felt such as Deck armor And then the slip sheet is not required . Personally I do not like a full vapor barrier such as full ice shield under metal.
local codes must be obeyed
in miami they want a nailed down sheet of tarpaper or synthetic material nailed with metal caps
then tar paper or the peal & stick
then if the next structure is under 20ft away a fireproof layer before the metal goes on
your rules/code maybe very different
Be aware, #1, that metal roofs are not fireproof. If you put a flammable underlayment between the metal and the plywood you potentially have a fire hazard. In urban areas near grass/trees/forests you must follow the WUI (Wildland Urban Interface) standards of the International Code Council. Bottom line, check with the metal manufacturer and have them tell you the proper underlayment to meet fire codes.