Which kill switch to choose?
You need at least a 4-pole switch (2 poles for the battery, 2 poles to kill the engine) but can also run 6 poles (2 extra poles to drain power from alternator to prevent power spike). A good 4 pole kill switch will cost around $40: (http://www.amazon.com/gp/product/B012UW1GAI)
A better but more expensive option (~$300) is to use a Cartek GT solid state kill switch (https://rally.build/collections/rally-build-fabricated-rally-parts/products/cartek-gt-solid-state-battery-isolator-kit), much safer as you don't have to run high gauge wires through the car (see video below for the comparison), it is also a much lighter option and makes it easy to install an external kill button:
4 pole kill switch install
Using a 4 pole kill switch like this one (http://www.amazon.com/gp/product/B012UW1GAI) 2 poles are used to disconnect the battery and the 2 other poles are used to kill the engine while running from the alternator. Make sure you use the appropriate wire gauge (4 gauge min to battery (http://www.amazon.com/feet-gauge-red-battery-cable/dp/B002DZF1MW), 10 gauge to ignition). The installation steps are as follows: