Indeed, character appearance doesn't effect gameplay.
Actually, the helmets do prevent slight damage, so it'd have to be a two shot or so for the head, though like I said, on rare occasions I'll be found with a helmet
There's nothing really that's NEEDED to be bought, unless it's weapons, or weapon modifications. That's a big way to save GP.
Only buying NEEDED weapons/weapon modification(s) will save A LOT of GP.
Suppressors are nice, yet they deduct from weapon damage, and have only a slight effect, which I find not to be worth buying, unless you're capable of buying the suppressor which enables your bullets to not give direction to your opponent from which they're coming.