Ridge vents and the necessary soffit vents use physics to ventilate an attic space. The air in an attic is warmer than ambient(outside) air because it is warmed by the house and the sun. Warm air is lighter than cooler air, so warm/hot air rises. The soffit vents admit cooler air to displace and allow the hotter air to rise. The ridge vent is placed at the highest area of the roof. That means the hotter air has easy escape to the outside. Adding anything---more roof vents, gable vents, or fans actually decreases the efficiency of the ridge vent system, which only needs Mother Nature to work no matter what the weather/heat/ conditions are. The reason some folks think ridge vents need help is because the system---ridge vent and correct number/area/placement of soffit vents is incorrectly designed of the soffit vents are partially or fully plugged. A musty odor can be caused by mold inside the walls, which could have been helped by a leaky roof over that room. Replacing the roof may correct the leak, but will do little if nothing to correct the mold growth/smell.

There are solar powered fans to consider too. On a home with a gable vent at each end??? perhaps the "self-flashed" type that is installed very near your ridge (on the sunny side to gather solar in the summer) and would draw air from the gable ends. Another type fits in the gable vent with a wire and remotely mounted solar panel. I think you need to spend $500 to get a good unit but they are simple to install... like putting in any roof top vent, perhaps a $250 job contracted out? Easier, cheaper and as effective as retro fitting a ridge I'd think. Later you could add the soffit vents which should help draw cool air up to feed your gable vents and fan outlet. For a retro fit perhaps putting 4" X 12" louvered vents in every second void between your rafters would be a good compromise of effort and effectiveness. (In some older homes with no soffit vents there may be blocking between the rafters so check to see if the soffit air can get to the attic before you do all the work.) Radiant barrier foil: Perhaps there's a twofer here, especially if it's difficult to add the soffit vents for some reason. The stuff comes in 4' rolls and is inexpensive..... $70?? for 500 sf. Since it may not be easy to install it completely, I'm thinking, first to install it on the south and west sides on the underside of the rafters, thus blocking most of the radiant heat from the hottest part of the roof or gable. Installing soffit vents on the cooler side should provide a cooling draft across the insulation layer to feed our foil chimney. I suppose a full radiant barrier and full set of soffit vents would do better but perhaps the above would give 80% of the benefit for about half the work? (I'm lazy)

I think you might be missing something but you've already pre-judged that roofer so move on and forget about using him. You won't be happy with his work from that pre-judgment. Sounds like he knows 'can' vents won't ventilate properly. So the roof isn't going to last as long. Possibly the manufacture of the material he plans on installing warranties their roofing materials with proper ventilation and most likely specifically spell it out in the installing manuals. If he doesn't meet some kind of minimum spec for the installation .. roofing materials manufacturer will have no warranties on that material, yet he still warranties his otherwise proper installation. I'd speculate,,, He wants competitive feedback so he prices future bids accordingly. Doesn't want to be cheap nor expensive probably. Normal thing for him to seek what his competition charges in competitive markets.

I'm still not clear what you're planning to do. Are you putting the insulation on the floor of the attic, or under the deck of the roof, covering the entire underside of the roof? If on the floor of the attic, you need some kind of baffles from the soffit opening up to and a bit beyond the top of the insulation, so that it doesn't get into the space and block the air flow. If you're putting insulation on the underside of the roof, you'll need baffles all the way from the bottom to the ridge vent so that there's a free flow of air under the insulation from the soffit to the roof. Here's a video that's of value to illustrate first putting baffles on the underside of the roof, before adding insulation on the underside of the roof.