Congratulations on the new battleship. There are many ways you can mount your transducer like Mikey said with a magnet "which I've never done", like Alpha said with a "Mad Frog, or RAM", and Jad2t recommending the Mad Frog.
I've used the Mad Frog, RAM, and also have used Lexell. The Lexell is an adhesive you put inside the hull of your kayak and set the transducer in and it adheres the transducer to the inside bottom of your hull.
I like the RAM arm the best of the three that I've used.
As for power it depends on what you're running. I run an Elite 7 CHIRP, and I have a 14Ah battery which gets me 2 to 2.5 day trips, and an 18Ah battery which gets me 3 day trips, all depending on what settings I'm running.
Most people who are running 4&5" screens are getting away with 5Ah or 7Ah batteries which are much lighter.
I build waterproof boxes for my batteries out of Seahorse cases
http://www.seahorsecases.com/
I would be careful going Lithium on a kayak. Kayak fishing can be a wet sport, and water and Lithium Batteries don't mix well, they are very volatile.