Be careful with the wiiceiver and solder it. It can lose connection and go full speed or brake whilr you are riding if it loses connection. I'd get torqueboards 2.4ghz remote.
I have looked on heaps of car battery websites. The only ones I could find were these: http://www.giantpower.com.au/45ah-12v-agm-deep-cycle-battery
Maybe your country has a wider range of batteries because I can barely find any.
Torqueboards ESC works with the wii remotes but the wii remotes have dropouts which can cause accidents. I would spend the most money on the esc, it's the ost important bit.
2 things with your build:
I'd go with a GT2B because that transmitter and receiver has a lot of drop outs and they can cause accidents because you don't have control over the board. You could always get diyelectricskateboard.com 2.4ghz controller which is just as small and much more reliable...