When you buy a ventilator, it may come with the blower already installed inside. It depends. External blowers are an option. If you get one, you buy a vent without an internal blower. (Or something called an 'insert' which goes inside a custom-built case/cabinetry.) Putting the blower outside puts the fan noise outside, mostly. In your case it may not matter that much being so close.

At that low a CFM, 6" might be more prevalent.

I would suggest speaking with an appliance specialist. If you are doing a direct vent ( going right outside behind the unit ) then you will still have noise with a remote blower. If you can then run the vent higher up the outside wall or go up to the roof, this will reduce the noise considerably and make t more pleasurable to use the vent system.

Good luck!