Any cheap Technic sets with a good variety of parts? I want to start building a module like GBC mini loop 001, but when I check the part list some are very hard to get. How should I get started? 

BTW, I got a very big set, Bucket Wheel Excavator 420555, and wanted to be able to do MOC's based on the parts in there. There are still many parts not included that I need for building this small toy. Any suggestions?
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.