My team bought a few Arduino cars from amazon that have worked well for us:
https://www.amazon.com/Elegoo-Four-w.../dp/B01DPH0SWY
It's $70 but that's pretty cheap compared to similar kits. The kit includes code and parts to build a car that can track lines with line sensing modules, can avoid obstacles with a ultrasonic distance sensor mounted on a servo, and can be controlled with a remote/IR receiver or phone/bluetooth module.
You write Arduino code in C++ not Java, but it's actually pretty simple and teaches you concepts that you can carry over to the code you'll be writing for your competition bot. Even if the car doesn't end up working out, the parts such as the dc motors, motor controller, sensors, Arduino, jumper wires, li-ion batteries and the charger can be used for any Arduino based project. Most of the parts are generic and replaceable in case any of them get damaged.
The big issues I have with this kit is with the instructions and code itself. The instructions while simple, aren't the best quality. They were originally written in Chinese I think and the translators didn't always do a great job. There are a few errors and the instructors/pictures aren't always consistent. You probably will end up making a few mistakes if you aren't careful, but I wouldn't necessarily consider that a completely bad thing since it makes your students think about the process and gives them experience debugging and fixing mistakes that will likely happen on your competition bot. The code that comes with the kit doesn't always work right out of the box and may require some tweaking. But since it's Arduino there are many coding samples you can find on the web that you can use instead if you prefer that.