Venting takes a systems approach.
Is there enough vent area at the eaves?
Is the roof sheathing set up for a ridge vent?

I would pay attention to Bozrath's warning about the condition of the
roof deck.
Include in any contract, a unit cost for replacement.
If it were my roof, I'd tear off what's up there now.

Also, look at Building Science Corporation web site.

TB
Charleston SC