Quantity   Component name
1 × ATtiny45/85 SOIC8-W
1 × NRF24L01 module (2x4 pin) cheap from China
5 × 0805 passives 10K resistors, 100nF capacitors - for decoupling on the board
1 × Normal/Shottky diode, THT 1N4148 will work, so will 1N5819
2 × Resistors for setting the power-down threshold 10K/10K will likely work, threshold can also be set in software (there's an ADC on the pin)
1 × Storage capacitor for power-while-shutting-down to be able to send that one last packet
1 × 5V low-current PSU, as required get a brand name, so that it doesn't fail
1 × USB-ISP for initial programming of the board You don't have one yet?