What's versatile enough to participate in all or most of the following: Classic Sports Car Series (b); Classic Muscle Car (a); Classic Supercar (iB); Historic Racing Cup (iB). I'm leaning towards the Chaparrel 2J but 4.5 million is quite a few credits.
The 2J will destroy the competition of almost any race you can enter it in. The acceleration and handling on this car are ridiculous. Its only weakness is that the top speed is a little lacking. The only time this really becomes a problem is on races that have very long straights like you would see in the Like The Wind events. Otherwise you can tune the power on this car up or down and cover a broad range of events.
I used a Dome Zero for classic sports car. For muscle, I used one of the prize cars (red off-centre one, don't remember the name). Didn't get to the others yet. I might have used the Miura for supercar if it wasn't so expensive. Can the 330 P4 or Chap 2D win the Historic races?