Metal roof & insulating without moisture

I purchased this pier & beam house with an enclosed lean-to like addition to the house on a slab. I planned to turn the back area into another bedroom with a mini split and the front area into a game room with a mini split. It has an old metal roof that is very gently sloped- not sure on the pitch. Some of the 2x8’s are span 36” + apart with 2x4’s across the top that the metal roof is screwed into. The metal roof is old with some imperfections and not water tight at the moment. My original plan was to put some sort of sealant on top of the metal to make sure there is no leaks and then 2” of closed cell spray foam underneath to give me sealants on both sides of the metal. I planned to put batt insulation below the spray foam since it is cheaper. I also planned to put Sheetrock for the ceiling. There is no attic space so I’m not real sure if this would even work?
If I replaced the sheet metal would I need to put plywood on top of the 2x4’s before I put the new metal on or could I put it directly back onto the 2x4’s the way it was? And could I just use batt insulation even though there is no ventilation?
I just don’t want any moisture built up in between the metal-insulation and Sheetrock.
Trying to figure out the right way (if there is one)!
Thanks