The most useful and powerful item in Cyberpunk 2077 is the Reinforced Tendons leg cyberware.

Why? Aside from making it easier to close distances in combat, it gives you access to 50% more of the game's content. Instead of running on the streets all the time, with it you can also easily run on rooftops.

Don't buy any weapons, cars, or cyberware until you get this. The game keeps feeding you better weapons and free vehicles, and all the other modifications just make you slightly better at fighting (which you really don't need if you spec Cold Blood). The only modification that can't be replicated with other items and skills is the ability to jump high. Double jumping is more useful than the high jump because of its flexibility in combat.