Ok, if you look at Lowrances then they have a model that comes with a "Totalscan transducer". It has side imaging, down imaging and sonar all built into 1 ducer and this sucker is about 10" long. It should fit on your trolling motor. If you are thinking about getting a Humminbird then find member Wayne P here (he's on almost every Humminbird question, giving the answer), this man can answer every and any question you could possibly have, he is the sharpest guy I know of on the many forums I frequent.
I don't know anything about Garmins and there's not a lot posted about them; but if you like them then make sure they have lake map coverage for your area, they are lacking in that dept.