Sorry to hear about this. I had a similar short ownership of an audi a3 2.0 tdi about 4 years ago which I had brought from a dealer. Within driving home 15 miles. Coolant light had come on... Topped up and forgot about it for two days until it came on again. Took it back, they then called me the next day and said that there was a water pipe leaking, so they replaced that.
Got the car home checked the water again and had noticed they had dropped a rad weld into the header tank as the water was now dark brown with bits in it. Lost all trust in the garage at this point.
Read up online and spoke to a few specialist about the issue and found it had a porous cylinder head.
Dealer luckily gave me my money back after about a week.