I've had the game for around 2 weeks, and thanks to the current seasonal events, I've been buying up cars like crazy. Just get a B-Spec driver up to around 20-25, pick up a Honda NSX Race Car from the OCD (I am currently using the Takata Dome NSX '03), and grind it on the B-Spec Spa Challenge. I know it says a Bob with a level of 35 or higher is recommended, but with the Spa, you don't need one that strong. My two best drivers I was winning with were in the 20's initially, and they are currently at level 33 and 31 and they win every time I use them. With the bonus at 200%, it's a quick and easy $4,000,000. I just push my Bob a bit in the first two laps, and let him do the rest on his own. Tune the car to 600PP, and put on Sport Soft Tires. He will not pit during the entire race, and you'll take the lead by the 8th lap if your Bob is at least in 7th-8th position. I think you can win this race fairly easily with just about any of the race cars available right now in the OCD for $750,000, but the NSX cars seem to be the easiest for me. The Spa is easier to win with a weaker B-Spec driver than the Serthe, and it takes around 27 minutes to win $4,000,000, while the Serthe takes longer (I think it's around 40-42 min).
I went on a Ferrari and Lamborghini shopping spree just a few days ago, picking up 2 of almost every model (some models I just got 1).