The reason this is the one component not to economise on: it is the only part that can take the others with it when it fails.
Most components fail by stopping. A power supply can fail by delivering the wrong voltage to everything connected to it. That is uncommon and it is not theoretical, and the cheapest units are where the protections that prevent it are thinnest.
It is also the component you keep longest. Graphics cards and drives get replaced; a good supply survives two or three builds, which changes the arithmetic on paying more.
A few practical notes for the purchase:
Use the cables that came with the unit. Modular cables are not interchangeable between brands or even between models, and using the wrong one is a genuine way to damage things.
Check the connectors your card needs, particularly if it uses one of the newer high-power connectors, and make sure the supply provides it natively rather than through an adapter.
Do not reuse an old supply in a new build to save money. It is the part that has aged the most and it is now under a load it was not sized for.
On your instinct: big is fine, cheap is where the risk is. Buy the wattage you calculated, from a unit somebody has actually tested.