I'm not with the rest on the wire thing... watch the wire you buy, try to get the type with the flexible sheathing and make sure it is oil and gas resistant for under the hood. Not sure but it might be called Automotive Wire but you will be able to tell as soon as you touch it, it will not be hard.
Buying Painless Wire is way overkill even for me.
NAPA has the type I am thinking of and a local Marine Supply Store where I live has it too, so a place like AutoZone or other Auto Store should have something.
Don't buy House Wire, individual wire or otherwise. House Wiring Type of Wire is not flexible enough for easy wiring. If you go to Home Depot watch what you buy.
12 gauge wire should cover almost all of your wiring needs up to approximately 20 to 25 amps, get some 10 gauge as needed for the lighting applications with 30 amps circuits.
Another thing I like to do and some say WHY... but run all of your grounds back to the battery, maybe a terminal strip near the battery to make the connections look neater. This method will reduce Grounding problems.