Lara can build a sizable arsenal during her time in Peru. Most weapons and weapon accessories are bought from Merchants, but others are obtained for free as rewards for side missions or simply progressing the main story.
Due to Shadow of the Tomb Raider’s focus on stealth-based gameplay, virtually any bow in the game will serve Lara adequately on most occasions. Some new weapons can be unlocked through story progression and side missions, so there’s little reason to barter for guns. The bow also negates much of the need for other weapon types, especially when combined with powerful special arrows and skills that allow Lara to quickly lock-on for guaranteed headshots and simultaneously eliminate up to three unarmored foes.
Weapon choice and upgrades may be more useful if you plan to complete all available side content in preparation for the final battle. In this case you’ll have more coins and resources to buy and upgrade non-default weapons. Likewise, with New Game+, you can choose from Lara’s entire armory right away and put saved-up resources into upgrades.
If you’re going to purchase weapons and weapon accessories, the Crow’s Hoard Skill reduces Merchant fees by 10%. Also note that the resources gathered from beetles and spiders using the Owl’s Meal Skill sell for exorbitantly high amounts and are much more common than rare animal hides, gold, and jade veins. As a general rule, damage should be the first priority when upgrading.