Metal roof over wood shingle with no underlayment

Hello, we just purchased a house. It has a metal roof over wood shingle attached to skip sheathing. The metal roof is about 10-15 years old and there is no underlayment. There is currently no ventilation in the attic. There is some staining on the wood shingle that we can see in the attic. There are no leaks. We've been told the roof needs to be replaced by some roofers. Others have told us it just needs better drip edge and attic ventilation. I think the gutters were overflowing and water may have been wicked up into the wood shingle. I'm guessing the water staining on the wood shingle could also be from condensation? Any thoughts on if this should be replaced or if it is salvageable? Thanks!