Volkswagen engine and servicing problems

Two years ago I bought a 2008 Volkswagen from my local Volkswagen dealer. I have had no issues with the car until recently when I went to start it and all the lights for the gears, PRNDS, were flashing and the car wouldn't start. It turned over, but would not start. I called the RACV, but they couldn't start it, or find what was wrong. After locking the car and unlocking it, the car started. I took it to my local Volkswagen dealer who tested it with their computer and couldn't find any faults and was starting ok for them.

A few days later the fault happened again when I left work, so I couldn't start the car. It started after locking and unlocking it. It faulted again a few days later and this time my trick of locking the car didn't work. I called RACV again and they couldn't find the fault or start the car. Battery tested fine. It was towed the next working day back to the dealer who kept it for a week testing it and couldn't get the car to fault. I took the car home and the same day it wouldn't start again. I tried starting the car several times without it starting, but when it did start I drove it to my local mechanic. He spent the next two weeks checking and testing it and found faults showing through his computer system. He tried several attempts to fix it without luck so I took it back to the dealer for the third time. They eventually found fault codes and believed that I needed a new genuine Volkswagen battery. They would not test the car further until I had a genuine Volkswagen battery installed. Their initial tests the week before confirmed that my battery was in good condition and did not need replacing, now they said my battery was dead and needed to be replaced. I reluctantly agreed to do it. Now, six weeks later my car is still at the dealer undergoing testing and they advise me that they have diagnosed the problem to be the ECU and it needs a new ECU costing $2740.

They advised that over the six weeks they have had my car they have spent eight hours working on it but will only charge me for four of that. In total they are asking payment of $3577 including fitting and testing. If they go ahead and order the unit it has to come from overseas and will take approx. three weeks to get here. They are now asking for their loan car back, even though I do not have my own car working and have not offered any option to provide alternative transport for the next three weeks should I proceed with purchasing this new ECU. I am a working single mum with a 4-year-old and living in a country town, so I cannot get around without a car to take my son to kinder or get myself to work. I certainly cannot afford to spend $3500 on my car and knowing that I will not be able to recover that money when I come to sell it. I just don't know what to do.