I have a stir bar in my 5 gallon kettle and it does a fine job. From my trial and error I would suggest using a little 120V desk fan with a dimmer switch/knob worked in. My unit uses a 12V fan that has some impressive power for its size, but the resistance you get from that volume of fluid can really bog down/trip up the motor when the PWM gets above 50%. There are some photos of it in action in my build thread: http://www.homebrewtalk.com/f170/120...90/index2.html
. With my motor, I get better performance with a small stir bar rather than the originally purchased 3" bar, again just too much resistance on that large bar. An egg shaped stir bar also seems to work really well. With your volume you probably need some pretty good torque to keep up.