I love this seasonal! Absolutely my favorite of them all at this point.
I started off by doing them in the BMW M3 with a tune that I pulled off of the tune wiki, and then I modified the tune a bit to fit my driving style. I found the races a bit too easy to do with it fully tuned, so I limited the engine power to exactly as high as the fastest car in each race and that's when it got challenging and fun.
I do these races a few times a day either in the M3 or the Audi A4. I'm in the process of tuning up the non 4WD Opel to try them again.