i usually use my parts for the flamethrower.
but i find it is very important to use it for your guns. you'll be able to shoot faster and hold more clips.
I was always wondering why I'd be in a shootout with someone (me hitting them and them hitting me) and i'd always lose.
once you upgrade during the match your bullets, you'll see what I mean.
I love flamethrowing people to death !
It really depends on how you want to play, for instance if you're looking for stealth then I'd recommend anything with covert - if you're looking for aggressive play then brawler and marathon runner are always good.
For a bow class, I use covert 3 and sharpshooter 3
Sharpshooter - Health back with each headshot, less pain wobble when shot (this really helps in a firefight) and much steadier aim
Covert - Can't be marked, don't appear in listen mode when crouching, start with a shiv on each spawn. The way I see it, since the bow does not register on the minimap when fired, you might as well make a stealth class out of it.
Sometimes I'll use covert 2 and sharpshooter 2 and use the extra 2 loadout points for bralwer (extra hits with a modded melee)
Give it a shot