What to Upgrade First
Gearwise, your first priority should be your quiver. Other gear can add a tiny amount of damage to your attacks, but never as much as a whole other arrow will. Especially as you master the Predator Bow, the more insta-kill arrows you carry with you, the better.
Skillwise, Charged Heavy Attack is pretty irreplaceable for taking down enemies with heavy shields, while the Enhanced Predator Bow skill is good for taking down… well, everything. It's arguably the most useful skill in the tree.
Later in the game, the two adrenaline upgrades can make tough fights easy, and normal fights effortless; letting you unleash your most powerful attacks right from the get-go.
Weaponwise- you're probably better off saving your drachmas. Upgrades are expensive, and you'll constantly be tripping across new wepons and gear, especially if you keep up with Reda's quests. Do hang on to legendaries so you can upgrade them when you reach 40, but otherwise save your coin for emergencies.
Addendum: The DLC for the game raises the level cap, so you might hold off on upgrading your lesser-used legendaries until you've worked through those as well.