The below parts list is more or less everything you need to get working on this project. Due to the DIY nature of the kit, the list is not exhaustive specifically in relation to the electronics. If you are building this on a breadboard or proto-board you may need more parts that I have listed below. However, the below does contain all of the main parts needed for the project.

Part Quantity Notes
400x150mm 12mm Plywood 1 This is for the base, any relatively heavy sheet material at least 12mm thick will do fine.
3D Printed Parts Set 1 3D Printing files found here.
Nema 17 Bi-Polar Stepper Motor 1 Almost any stepper motor available for 3D Printers should work file for this. The electrical characteristics are not critical, however, the motor should have a 5mm shaft with a flat spot.
HC-SR04 Ultrasonic Sensor 1  
Regulation Table Tennis Ball 1 Or any light ball of 40mm in diameter (heavier ball will change how the system responds).
PCB 1 The PCB is optional, the source for it is here so you can get your own made if you like. Otherwise you can make your own with proto-board from the schematic or assemble it on a breadboard.
Arduino Nano Compatible MCU 1 Any Arduino should work for this, however the PCB has a socket for a Nano.
DRV8825 Stepper Driver 1  
22uF Capacitor 1  
12V 2A DC Power Supply 1 If using the PCB, ensure that you have a 2x6.4mm centre positive barrel jack.
4 Wire Female to Female JST Cable 1 This is for the motor.
4 Wire Female to Female DuPont Connector Cable 1 This is for the sensor.
M3x10mm Socket Head Screws 4 Motor mounting hardware.
M4x10mm Wood Screws 12 Mounting screws for PCB and base pieces.
M5x25mm Socket Head Screws 2 Linkage mounting screws.
M5x20mm Socket Head Screws 1 Linkage mounting screws.
M5 Nuts 6 Linkage mounting nuts.