Not sure why this is in General Physics, but oh well. The answer to most of your questions is:
Whatever's cheap, while still being capable of doing the job.
Metal: probably tin or plated steel, something that's cheap, easy to work with, and won't severely oxidize when the lighter is used, or worse yet, melt under the likely flame temperature.
Plastic: polystyrene or polypropylene or something similar that'll resist the pressure they put the butane under, and won't be dissolved by the same lighter fluid. Oh yeah, did I mention cheap?
The Zippo (and similar lighters) aren't designed to be disposable lighters (they also use naptha oil, and a wick instead of liquidified butane). They're designed to last a long(ish) time, and possibly be used even as a fashion accessory. You're paying for all of the above, and probably for the brand name and premium as well.