The 12V model will be suffecient for your 18' toon, but, if you have the money, and plan to do a lot of trolling, I would get the 24V. With the 24V you can troll around for a long, long time, unless you're running wide open. Yes you do have more expense for batteries, but I would be most concerned with the battery placement. Do you have the room for 2 deep cycles? Also, consider the weight. If your two TM batterys, the starting battery, and you at the helm, are all on the starboard side, you are probably going to be listing to that side whether your wide open or just sitting in the water.

As for adding a kicker to a toon, I can't say I've ever seen it done. I personally think it would be overkill. The good 24V trolling motor will get you back to the ramp if you ever need it to.

And, welcome to Crappie.com!