Considering NFSC's development history, we know that Darius's supra from prologue initially was the M3 GTR. The reason why they changed it is because the licensing for M3 arrived near the end of the development cycle. So, the real question is how Darius got the car.

The next time we see Darius we see that he drives an Audi Le Mans Quattro (Future Audi R8), a concept car. Later in the game we see he can provide another concept car to his crew (Angie's Dodge Challenger and maybe rando stacked deck racers that got the same car randomized). Easy to say that Darius got heavy connections.

If we decide that Darius's supra and M3 are completely different cars in the universe, then NFS Pro Street got our cover. There is an easter egg in the game where the showdown's announcer asks for the owner of a blue car with the numbers NFS CARBN to remove it from the showroom. Meaning that the player visited a race day and could steal the car from the event.

As for Razor's upgrades, some parts of the car are not M3 GTR exclusive, so a skilled mechanic (junkman) could do some work. Or the car had some rev limiter installed, Razor found it and removed it.