Revision history [back]

It doesn't really matter which you learn, although Humble is current and Foxy is not. There are specific differences that will complicate using Foxy or Humble but for learning the concepts and getting experience, it's no matter.

Regarding if you should buy a smart cart or not....?? Not if it's this one. https://www.tackeria.com/contents/en-us/p3408_Smart_Cart_with_Flat_Free_Tires__.html

You can get a lot of learning using Gazebo/RVIZ with only your PC and HW. On the other end of the learning spectrum is building your own robot from scratch and developing your own HW drivers in ROS and making it work. Buying pre-made HW that already has it's own drivers and functioning ROS build seems like the worst way to learn it. You won't be forced into the details that really matter. You'll have a toy and platform for further development but may not learn the basics at all.

I say create your own simulated custom robot using the Tutorials and once you understand how ROS works, you then think again about HW.