I would not make much of a distinction in metal vs plastic. Some may say that metal is louder because of the bore shape. Some may prefer a metal emb plate. I'd say that it's all in what you are used to after you practice the beast.
I agree with buying an instrument with a known name and from a source that has some sort of guarantee. I'm not a piccolo playing fan, even with marching...LOL! When I had the fortune of 1st chair, my 2nds always liked me because I would happily give them the piccolo parts with the solos Fortunately, I moved during high school to a school that focused on the concert season and didn't march, so I got a reprieve from my "can't walk and chew gum at the same time" coordination (or lack thereof...). I greatly admire anyone who can play a good pic. (especially if they are marching at the same time). Good luck with your search for the right instrument.