I would: 1 Start learning about ROS, 1a run some basic tutorials, 1b run a virtual robot in Gazebo (simulator); 2 Buy one of those raspberry pi based robot kits

I would: 1 Start learning about ROS, 1a run some basic tutorials, 1b run a virtual robot in Gazebo (simulator); 2 Buy one of those raspberry pi based robot kits; 3 Buy an intel d435 3d camera (if you can afford it); 4 repeat what you did in gazebo in the real world on your pi-robot.