you only need to run the +ve cable back to the boot, and then earth the battery onto the shell in the boot
i'd get 6m of cable to be safe, 4gauge is fine (MUCH cheaper from a welding shop) if not halfrauds have it
install a cut off switch if you want
terminals etc from a car stereo place
on a 172/182 you need to extend the starter motor cable by about 12-18inches approx
i tend to install a distribution block into the std fuse box, but this isn't needed, i just do it to make installation easy
there's a huge current draw from the starter motor, i normally use 120amp cut off switches
you need a battery box for your std battery, or a redtop race battery (the little ones) is about £70-80