https://store.ctr-electronics.com/
I would peruse all these guys and look at their products. It's an amazing source of ideas.
Vex has a free IDE and virtual robot sand box. So you download the software, build your bot then use Scratch or c++ to program it to operate in a virtual sand box, or virtual challenges. It's like a video game, but you have to build it. It has a lot of tutorials and is very easy to get started with.
What about this kit for a bit?
Go Kart Buggy ATV Snow Sand Snowmobile Rear Axle Track Assemly for Gasoline Motor 23.6" Rear Axle Length Rear Axle Track Kit (US Stock) https://www.amazon.com/dp/B09LV7TWX3/ref=cm_sw_r_cp_apan_glt_i_XEN0GNQQ3TY7XXBJ7TXV
Now that looks more like what you'd need to go build a bot for metal detecting...lol
regards,
LouisR
LouisR