The below table presents all of the available equipment upgrades (the "Gear and Upgrades" tab and "Artifacts" of the barter window). It also gives you some hints on which upgrades are the most useful.
Note! What is missing from the table is the information on the price of the individual upgrades, because they depend on the difficulty level that you selected. Also, it is worth knowing that some of the upgrades become available only later into the game so, try to visit merchants (the ones in taverns), each time after you complete a main chapter and check out on the prices of the upgrades.
This gadget is required to obtain a dozen-or-so various collectibles in the game world (paintings in the Court of Montonessi category). I recommend that you buy it immediately, thanks to which you will not have to return to those of the items that you failed to collect.
The vest slightly increases your health bar. It is not too expensive an upgrade although, if you play a cautious game, Garrett's life will be threatened only several times throughout the campaign (additionally, these are not induced by Garrett himself) .
Ovengut Tonic Water
This is one of the most expensive upgrades (it is almost 10 thousand gold pieces at the Master level!), which increases the health bar and the Focus bar. Personally, I recommend that you invest your gold into cheaper and more useful upgrades, first.
This is a two-level upgrade, which increases the effectiveness of the lockpicks. It is immensely useful, because Garrett needs to pick locks quickly, at some points, before he is detected. Try to unlock both levels pretty early into the game.
This gadget is VERY USEFUL and you use it to open venting shafts, as well as to remove plaques (one of the collectible categories). Try to buy it immediately.
The role of the quiver is to suppress the broadhead and serrated-edge arrows, after being fired. Due to its low purchase price, you can buy it straight away although, even without this one, you do not need to worry that the arrows you shoot, will alarm the enemy automatically.
The bond decrease the prices on all the basic resources by 25%, and therefore it would be a good idea to unlock this one early into the game. This will allow you to replenish your resources at lower cost.
This is a three-level upgrade, which increases the damage that you deal with the club. If you want to complete the game, while operating from shadows for the whole time, you will find no use for this upgrade, especially that you can eliminate opponents, quietly, in melee.
After being used on the boots, it decreases the noise that you make, while walking over noise-producing, surfaces, e.g. water or broken glass. These are quite expensive but, if you have enough gold on you, it is a good idea to buy that, because you will be running into the "loud traps", mentioned above, nearly in each mission.
This one decreases damage that you take from fire and the traps that you spring. You can easily ignore that, assuming that you are not going to complete the missions in haste. Each one of the blazing areas can be disarmed.
The function of the feathers is to increase damage that you deal with broadhead and serrated-edge arrows. You do not need it because, if you operate from shadows, you will be attacking nobody. What is even more, even if you decide to eliminate the monsters that appear here and there, throughout the game world, you will use a different type of arrows, anyway.
This is a two-level upgrade, which increases your arrow capacity. Try to unlock, at least, the first level, so you can bring more broadhead, water and rope arrows along.
Bow Shot Strength
This is a three-level upgrade and it increases damage dealt by arrows . If you are trying to beat the game without raising any alarms, you may find this upgrade useful only while fighting monsters, although you can also walk around them, without any problems.
This upgrade is very useful, because it increases the height that you can safely jump off. Since Garrett often moves on rooftops and ledges high above, you will appreciate this upgrade, nearly, immediately after you buy it (preferably early into the game), after you find out that you do not need to look around for ropes, ladders and other means of dismounting elevated places.
Thanks to the coin, you can increase the level of available focus. You can buy it because, even at the highest difficulty level, the coin is very cheap.
This gadget is VERY USEFUL and you use it to sabotage boxes that power various traps. It is recommended that you purchase this upgrade immediately, especially that it is impossible to walk around some of the traps and, as a result, you may miss some collectibles and secret areas.
The satchel allows you to carry more food and poppies around. I recommend against buying this one, especially that it is not too cheap.
This is one of the most expensive upgrades (28000 gold pieces at the Master level!), which decreases, considerably, the risk of being spotted by the enemies. It is very useful if you are playing t the highest difficulty level and/or you enabled additional modifiers. This, however, does not change the fact that it's price ids ridiculous so, start saving up gold only after you obtain all of the more important upgrades.
The Medallion decreases the risk of taking damage from projectiles fired by the enemies. I recommend that, if you decide to buy it, you obtain it only towards the end of the campaign mode, because it will make things easier, during the encounter with one of the bosses.
Salt increases the amount of health points that you replenish, after you eat food. This is not too helpful because, if you play carefully, you will be losing no health (the only exception are the smoke-filled areas, but you should have sufficiently much supplies on you, by that time).
This is a two-level upgrade, which decreases the time that you need to draw the string of the bow. This is not a priority because, if you operate from shadows, you will always have plenty time to [re[are your shot.
This is a two-level upgrade and it decreases damage that you take. If you want to beat the game, while remaining in shadows, you will find no use for this one.
This is one of the most expensive upgrades (28000 gold pieces at the Master level!), which expands your health, and focus, bars considerably. I recommend against this one, because at the price of this upgrade, you can afford buying lots of other, more useful ones.
A document that decreases the price of poppies, offered by merchants, by 25%. This is a useless upgrade, because you can often find poppies while completing missions or exploring the city.
The most useful equipment upgrades
Arguably, the most important are the three additional gadgets, which are offered by merchants and you should purchase them at the first opportunity (if you do not have enough money on you, take a look around the city for loot). The Wrench is on offer right from the very beginning, i.e. since the first visit in the city, after you complete the first chapter. The Razor appears in the merchants' stock after you have completed the second chapter and you can remove the unique paintings, from their frames. Wirecutter becomes available also after you have completed the second chapter. This is even more useful a gadget than the razor, because disarming of traps will allow Garrett to avoid taking serious damage, or even save him from dying.
In the Tools & Upgrades category, one of the most interesting items is the leather padding, which considerably increases the height from which you can jump down from a rooftop or a ledge and decreases the amount of the unnecessary damage that you take. Apart from that, I recommend that you obtain the Leather Oil sufficiently early into the game (quieter walking over loud surfaces) and, at least, the first level of Quiver Capacity (increases the capacity of arrows that you can take with you) and two levels of Lockpick Quality (this way, all of the lock varieties open faster).
It is best to start your purchases in the Trinkets category by buying Basso Bonds, thanks to which arrows are going to cost you less, throughout the rest of the game. It is also a good idea to take several upgrades offered on discounted prices, like the Lucky Coin (more Focus). Moss Quiver (suppresses some kinds of arrows) or the Warm Vest (more health points). Before you meet up with the general, in chapter seven, you should consider buying the Crosswind Medallion, which will allow you to avoid projectiles more effectively. Towards the end of the game, you can consider obtaining the expensive Spectral Aspect, although you should make sure that you have already bought the remaining, more important, upgrades.