Electronic parts can be delicate, and a greenpower car, especially in the rain, isn't the friendliest environment. Every now and then new components will be needed. Unfortunately Greenpower doesn't stock them, but they are cheap and readily available online from sites such as Amazon, eBay and if you can wait a couple of weeks, even cheaper from AliExpress.
The parts most likely to need replacing are the Bluetooth Module and Arduino. Below are some recommendations for how much parts should cost and where to find them
Search for HC-05 Bluetooth Module.
Amazon: Expect to pay £6-£8
eBay: Expect to pay £3-£6 from UK sellers, <£3 from China
Search for Arduino Nano 328. There are two variants of the nano, the 328 and the 168. Both will work, however the 328 is the variant this documentation covers. For more info check the 'About the Arduino Nano' section.
Amazon: Expect to pay around £7
eBay: Expect to pay around £4 from UK sellers, £1.50+ from China