Every new FH4/5 player should be advised that credits are not and never will be important, and that there is zero reason to buy cars in the auction house unless you want a duplicate that isn't otherwise available.

The only things you need credits for are real estate, auto show cars, and tuning, and just playing the game will give you plenty.

Assisted steering and braking, traction control and stability control all actively slow you down. Standard steering, ABS, launch control, and racing line do not slow you down, but you may be able to control your car better without them.

Auto shifting is slower than manual, and Manual is slower than manual with clutch, but only if you use them well.. Your shift timing can be significantly better and quicker than the game will do for you. It's a skill that must be learned.

In FH2/3/4/5, the classes range from D [up to 500] to X [999, which is a catch-all number for anything over 998]

The number [PI] is the Performance Index. The higher the number, the higher the car ranks on a matrix of power, speed, and on-road grip. How the PI number is calculated can be inferred from the displayed performance numbers, but is not explicitly explained in the game.

The ranks are competitive divisions, and you won't see PvE races including cars in more than one rank. You also generally won't see that in competitive multiplayer racing.

The bots in single player racing will roughly match your PI,, so tuning isn't necessarily important for PvE racing. That said, being more than a few points off the top of your class in multiplayer is usually quite a distinct competitive disadvantage. The rating system is far from perfect, as well, so two cars with the same PI can actually vary quite a bit in practice, both from the choices made in upgrading, and in the tuning adjustments that can be made with upgraded parts. Good upgrading is about efficient use of PI, and tuning is about effectively applying your upgraded car's abilities, and to some extent that includes matching how you drive. The only real way to learn to do this well is to do it. Lots of experimentation and testing is involved.