The most expensive car is the Koenigsegg Regera (worth 1,701,700 credits). You can get the "Ride The Jewels" achievement using test drive (you do not have to buy the car). This can be done at the start of the game, after having unlocked the Street Race HQ. Go to "Catalog" -- "Koenigsegg" -- "Regera" -- "HQ". Test drive the car. There is a timer during test drives. If you cannot reach the 2 km in a single test drive, you can do another test drive and it will add towards the 2 km total.