BMW demo dilemma

Back in June I visited a BMW dealer here in Brisbane and was given a test drive in two demonstration cars, I enjoyed the experience and was prepared to enter into negotiations over a potential purchase, the parameters being how much I was willing to spend per month. The salesman could not put a deal together at the time but later rang to tell me he had a car to show me and asked when I would like see it, to which I said it would be the following Saturday. He would not be there that day he said, but another salesman would take me through the purchase. He also requested I sign an order form and pay a deposit to secure the vehicle subject to me seeing it on the Saturday. I agreed and paid $4000 on my credit card. When I went to look at the car the following Saturday I became concerned that nobody in the dealership was aware of the test drive. Eventually I was shown a dark grey X5 that was out the back of the dealership and told I could take it out if I wanted to. When we got to the X5 we were astounded to find that it was filthy dirty, it looked like someone had been sick over the front seat, and that the back seat was covered in the remains of someone's lunch. We took the vehicle round the block but were generally very unimpressed. The next week the salesman rang to ask if we were happy, as you can imagine I expressed our concerns about the presentation and even asked if he had another vehicle to look at, as I did not feel at all confident about this particular X5. His response was to argue that it had been a small problem on their part but they would clean the car and have it ready for a further inspection the following weekend. We then had some discussion about the fitment of a different set of wheels, which he promised would be done by the time we came to look at the next weekend. But on the Friday the salesman rang to tell me that he couldn't get the wheels, but that the vehicle was ready for collection, I told him that I wanted to see the X5 again before making any commitment. Before I went back to the dealership I went to another BMW dealer and asked them about how they demonstrated their vehicles and the sort of presentation a prestigious mark like BMW would expect, as I suspected they were aghast at the treatment I had received, but kindly also ran a check on the X5 I was being asked to purchase. They confirmed my worst fears in that the X5 was registered as a "service loan" car and had never been a demonstrator. This explained the condition of the car when we viewed it as it had clearly just been returned after loan. I then called the dealership and told them that I would not be proceeding with the purchase because the car had been misrepresented to me as a demonstrator. The manager rang me on the following Monday and we had a long and at times difficult discussion. He claimed that there was no difference between a demonstration vehicle and a service loan car; he even tried to tell me it was a "service demonstrator". We eventually agreed that our interpretations on the definition of a demonstration vehicle were very different and that we were not going to resolve the matter there and then. I then received a letter from their Chief Finance officer demanding full and final payment within seven days and quoting the 'contract' I was asked to sign to secure the vehicle as a legally binding document. As you can imagine I was somewhat annoyed at their actions. No one would talk to me at the dealership so in desperation I rang BMW Australia and I explained my situation to a customer service person, and she promised she would get the dealer to call me to resolve the problem, but she received the same treatment as I had. I believe the dealer has behaved in a very dishonest way and has attempted to sell me a car without full disclosure of its history; additionally they are now trying to enforce a contract into which I believe I was 'coerced' into signing in the pretext of securing the vehicle prior to inspection.