I did the VGT and Gr.1 Weekly Challenge races, and won both.
That gave me both a 4* ticket (DM) and 6* ticket (WC), which both paid out a helpful 500,000 Cr., giving me the 33M I needed to buy the two cars I wanted from the LCD - which, somewhat unhelpfully, arrived at the same time.
I kind of knew that was going to happen, hence why I set myself the target of collecting 33M.
So I bought the Barker Tourer and the Shelby Cobra, the latter being the final 20M car in the game that I needed.
Those two cars alone represented just over 50% of the Grand Total required for me to buy every car in the game; I still need to buy 39 cars totalling 31.6M.