I advance the UCD quick by doing 5 seasonal races for ~$1,500,000. If I have cash, I go to Licenses and skip through 5 license loading screens. Takes about a minute or two and refreshes the entire UCD. So trading a $60k car for a $400k doesn't matter when it takes 5 whole minutes to make $400k. Though I believe the S7 is more rare than the Spirra.
I feel like I'm the only one with a '91 Lexus GS300.