1. Get the dealer to fix it. It should have 3 months warranty at the very least.
2. Contact DFT/consumer affairs to see what else can be done.
3. Learn from this experience, don't trust anything the salesperson says. You got shafted. Things like checking to see if there was aircon is a given, take someone who knows what to look for next time. When you buy a used car, budget an extra $2k on top for repairs. If you can't afford that, then you can't afford the car.
Welcome to used car ownership and buying from a dealer. You are lucky that you have this SAU community to help you out, so don't be so harsh when you make silly mistakes like not checking the aircon. People will attack you, welcome to the Internet. Even my mum knows how to check if the aircon works.
Good luck.