[Mod Edit - advice should be constructive; this thread is not an invitation for a debate. Let’s let the community members try things out for themselves to determine whether they’ll find enjoyment from it. - MM]
As soon as you’re allowed to buy your own cars, buy a Class D car for each racing series and go up a PI class every time you level up. I also recommend picking a different vehicle for each exhibition. Its the best simulation of progression in a game that for the most part has none and will keep your main playthrough unpredictable.
You can make a lot of money by studying the market in the Auction house. You can buy some cars for low amounts of money that sell for millions. Then whilst the cars are in auction you can do 3 laps of the goliath in the Alfa Romeo Giulia FE with all of its skills, and make a load of money that way too… about 250000.
For racing: Start out slow. I know the games throws a lot of awesome cars at you for free, and its tempting to use one of these awesome supercars. But the fact is: Fast cars are much harder to drive then slow cars, and if you chose a fast car, your opponents will get one as well. So start your career with a VW Golf and leave the McLarens for later.
For PR stunts: Faster is better. Try settling on a good car for every kind of task. Avoid doing them in winter, they are much easier on dry roads.
A good early choice for road pr-stunts is the Mosler. They are dirt cheap in the auction house, have good top speed, good corner speed and a good acceleration.
For everything offroad/dirt the RS200 is what you want to have. Quite pricy though.
Never drive an untuned car. Best is to learn how to do it yourself. Or find a reliable source of good tunes.
Get accustomed to the playlist early, easiest way to get money and cars. And many of these cars are only available via the playlist. If you miss them, it might take years till they show up the next time.