I'm wondering how many credits is enough...
I'm not aware of any easy way to make a list of which legendary cars I don't have? I can check the highest value ones easily enough I suppose. I know I have most of the legendary cars, e.g. all of them that are currently available.
I know the total cost of all the cars is a little under 500m, from the thread about that. I think the game tells you somewhere what the total value of your cars is, so allowing for some duplicates, I could take the difference and allow a bit of a margin.