I’m not the best person to answer this as I’m not much of a Lego person. I can follow build instructions but I’m awful at creating my own, so I don’t really know what’s useful.
That said, you would need up to four motors, and sensors. I got mine off eBay. Then you need to get some LEGO Technic building blocks.
If you look at this document you will find a list of all blocks (including their block numbers) that come with the default Mindstorms EV3.
Sometimes you can get a kit off ebay that has just the building parts (there’s one at this very moment, it sells for 90$ but only has the building blocks, no sensors, no motors. Sometimes you can also get a cheap set from someone who killed their Lego brick but I’m not seeing one at this moment.