As Cees says, you probably need to decide: either 250SL or 1976....
As for the datacard: I am not sure how it works elsewhere, but in Europe:
1. You go to any MB dealer. You show proof of ownership and request a datacard.
2. They take a copy of your ownership document.
3. You get a data card within a few days in pdf format via e-mail.
You do not need any special e-mails or contacts to Classic Center. Same with parts ordering.
Of course there is a huge value of someone like Tom - priceless!. One of the reasons is that, i.e. it was me who had to teach the dealer staff on how to look for parts, what are the correct parts etc. for older cars and Tom knows all of this and is of great help (also for me in Europe!). But for a simple thing like data card - you just need a local dealership, I think, also in Australia.
It is also a good idea to establish good relationships with local dealers as you will need a lot from them on your journey. Discounts among those needs are one of the top ones.