There is no better way to learn about robots and coding than building and programming your own. This build-it-yourself Rover kit might give you a few ideas. It is a rover with 6 independent wheels with Raspberry Pi on board. It is inspired by the ExoMars rover.
This project is currently on Indiegogo. According to the maker, those who back the campaign will get the plans to build and 3D print a rover themselves. It should cost you about 500 euros to build something like this, depending on where you get the parts.