the m2 should include all the parts to connect it to the motherboard properly, but you will need to supply your own wire for connection to the battery.
why not get it from the mp3car store, and help support the forums?
for battery connection wiring, i like to recommend the amp wiring kits at walmart-- they are dirt cheap, decent quality, and will include almost everything you need for a install.
for the amp wattage, you do not need to factor this into the power supply calculations unless you choose to power it off the m2. the power supply calc's are mainly to make sure that you do not overload the computer power supply.