The Cheapest upgrade is the 312mm upgrade.
As Dom said you will need the carriers, as you can use your exsisting calipers. or if you have OCD like me lol then you will change the calipers too, now I only did this because I wanted to ensure that I refurbed them and there was now corriosion and the paint coverage was correct

And as I was wanted to do a brake fluid flush as I dont know if the car had ever had it.
Dom did miss that if you are upgrading to 312mm you will also need the splash guards
OR you can cut your exsisting ones but again because I have OCD i got the upgraded ones from germany

This was my end result before they were fitted.

Before upgrade (Please note the wheels in this pic are 17" running the 288mm discs and old carrier/caliper setup)

And this is after the upgrade, only been on the car a week so not fully tested the brakes yet and the discs/pads are still wearing in. (Please note these wheels are 18")

