Mike I prefer transducers inside so they don't get damaged by the trailer which happens if transom mounted, or if you beach her. Problem is you don't have access to the inside of the hull without cutting the deck, apart from one small area inside the rear box sections were the A frame attaches to the hull. If you are brave then one of these mounted just behind the console could be an option to give you access to the inside of the hull.
As for batteries, if you have the standard console you are a bit short on space for a second battery, however these are worth a look as they give some serious clout to start an engine. We use one of these on the yacht to start the diesel engine.
http://www.myboatsgear.com/mbg/product.asp?prodID=1719
Pete