Which charger should I buy

The following: 30W USB‑C Power Adapter - Apple

FWIW, get the 61 watt especially if you use a dock

with power pass through. When connected to the

power supply, they will deduct their power requirements

from what the supply can deliver.

Example, I have a Belkin Multimedia Dock. It will use

15 watts when the power supply is connected (only 7 watts

when powered from the MBA). With my original 30 watt

M1 MBA supply, that left 15 watts for the MBA which is only

adequate for light tasks and if doing heavy duty tasks, the MBA

would switch to/use battery power to make up the difference.

Also, if the M1 battery level is below the 80% mark where "fast charge"

takes over, the MBA will not charge if powered through the dock

and the 30 watt supply had to be connected directly, thereby loosing

my external display.

Using the 61 watt supply solved the issues completely.