I have the Humminbird 180 which has the widest coverage (90°)of any of the lower end units and cost $140.39 from Amazon with free shipping. Comes with a display unit, mounting hardware and the transducer.
http://www.amazon.com/Humminbird-Piranha...1469&sr=1-1The wider coverage will not show the bottom better but it does show fish on the sides of the boat and which side they are on. Most any Fish Finder will improve your fishing, as it lets you see how deep the water you are in is and find the right structure to fish (like flats and drop offs).
The only other items that you need will be a battery and charger. You can get the Humminbird 12 Volt 7 AH battery with charger also at Amazon for $35.16 also with free shipping.
http://www.amazon.com/Humminbird-7700281...1368&sr=8-1Smaller batteries will work but this one is probably the best one for the project.
My Fish finder is mounted in a special scupper under the seat and is open to the water. However other people mount theirs other ways.