I'd actually say looks is a bit more important than performance because its much cheaper to change out internals than externals. Also since you're gun is gonna be collecting dust anyway, you might as well get a good wall hanger that you can keep up internally when you're ready to play. I'm pretty sure most of us here care more about looks than performance, otherwise we'd playing laser tag or paintball.
Other guns are just as customizable, but the parts are a bit harder to find because they aren't as generic.
I think the KA M4 is a plastic body gun. Most cheap guns are plastic. Another reason I strongly recommend waiting.
Guardians of Asgaard - KF32
VFC H&K 416C
VFC KAC SR16 JL Billet SBR
TM FN Five-seveN