I suggest to not buy any car at all. I had 3.5 million from the digital pre-order (2.5 mill) and demo (1 mill) and I'm now over 4.6 million from all things combined not too much playing time yet (working week & busy weekend). The game rewards you with gift cars a lot of times and it would be really annoying if you first buy a car and then later get the same one as a reward. Most expensive cars cost about 1 million (all VGT's) with 5 cars even costing more, the Veneno is the most expensive one with 3.6m , the Vulcan 3m, Veyron 16.4 2m, La Ferrari 1.67m , One-77 1.3m.
You can get free cars from any of the following :
* If you play daily and drive more than the daily mileage workout
* career mode - complete a row of mission challenges in both driver school and mission challenges
* career mode - get a gold star on all track challenges for each of the tracks
* your birthday
The best strategy is to just play the game with the cars the game gives to you. At the point you start to have more remaining credits than the cost of the cars you still lack in your garage, then start buying them.
As for the individual cars, within Gr.3 or Gr.4 the cars all pretty much feel the same to me. I'm sure if I go more into detail on setups I will find differences but from a "just hop in and go" they all pretty much sound and feel very similar to me, especially when using the road camera view with the speedometers.