I had a Minn Kota Endura 44lb (12V) on my DB and that was sufficient for trolling and side drifting. I recently upgraded to a Riptide 55 (12V) so I could play around in the sound.

I wouldn't go much less than 44lbs. I like the 12V systems because when I'm side drifting one battery will easily last all day and I don't have to deal with a second battery all the time. For heavy duty trolling I'll take up to 3 batteries.