Hi Peter
Great to have another Melbourne car. I also recently joined and have done exactly what you are looking to do.
My car was originally South Australia registered - and when I bought it the agent said there wasn't a great deal (read no) history. I used that to drive the price down a lot.
I then called the Mercedes agents in Adelaide and a couple of them looked back at their records (for the original registration number) and gave me some of the detail that they thought they could without telling me who the owner was. One of them contacted the owner (who was still a customer with a modern car) and he then contacted me. It was great. He told me that he had given the agent in Melbourne some of the records - which the agent then 'remembered' and I got hold of them. That was a month after I bought the car. I think the bottom line is keep the pressure on the dealer.
The only two other sources are Vicroads (don't expect too much help there) and I would also try Shannons or Lumleys (Insurance) as there is a good chance one of them covered the car recently. One of them may put the previous owner in touch with you.
The most valuable record - other than service record - is the data Card. Every car came with one and it gives a HUGE amount of information as to the car build and original equipment. Have a look at this link -
http://www.sl113.org/wiki/DataCard/DataCards.
I ordered mine from the US - one email and no charge. I doubted if the US would have Australian records, but it came straight back with all the data - and a translation.
Good luck.
Richard