GTR Motorsport save up and use that one, thats a killer, its mostly a last lap winner, meaning with all the AI braking and slowing down to let you catch up, you'll probably get first on your 4th, 5th, or 6th lap.
Most of these cars are LMP cars so they'll kill you on acceleration, but you can cut seconds off with corners in the GTR.
I know you said 600,000 credits, but the Toyota Minolta 88 is the best purchase. You'll win every race by 20 seconds using that.
If your going for the 600,000 credit limit, your kinda limited, maybe 1m would be a better bet for cars to buy. Most GT3 or GT500 cars are 950k and above, and these cars are a lower class than LMP ones.