Any cheap Technic sets with a good variety of parts?
In answer to your question &quot;I want to build a Great Ball Contraption and have LEGO set 42055, what other LEGO sets do I need to buy?&quot; the answer is: None. You've got all the bits needed, even a motor.
Instructions to make a Great Ball Contraption from LEGO set 42055 are available for purchase for about USD25, and advertisement for the instructions can be see on YouTube. The are other instructions available - like https://www.bricklink.com/v3/studio/design.page?idModel=140147
The PV instructions are just one of dozens of alternative 42055 builds.