An external blower with a short run would probably not be quieter than an internal blower, but that could depend on the design. A large slow fan would have lower blade tip turbulence noise, but the baffle noise and duct noise would remain. An in-line silencer (monster "muffler") would quiet much of the external blower noise, but they are large and might be considered un-aesthetic mounted over the hood in the kitchen.

Cooking cfm required depends mainly on the hood aperture, which should be big enough to capture the rising and expanding cooking effluent plume. Larger hood, more cfm; smaller hood, escaping moisture, grease and odor. Be sure that there is enough make-up air, or the actual cfm will be much lower than expected. In any case, the cfm will be lower than rated because there are still pressure losses in the path between aperture and exit. With open windows one might get 700 (wild-guess without a fan curve) with a short length of 10-inch duct and baffle filters.

kas

Thanks, Kas. The hood would be 42"x24" over a 36" range. MUA should not be a problem because we open the kitchen window when we are home pretty much year round. (Plus, our 52 year old house is not exactly air tight. No fireplaces either.)