If you do not own this game yet, you will. And if you don't get it in early access, you'll be kicking yourself for a long time.
Assetto Corsa - My personal favorite racing simulator. The only thing that comes close to how fun this game is, is Richard Burns Rally. We're talking incredibly detailed content, a burgeoning modding community and soon to come, the first ever laser-scanned Nurburgring Nordschleife. This game will replace Forza, Gran Turismo, rFactor, Live For Speed once development is completed.
We may not be able to alter the default cars just yet, but I'm damn sure that Kunos will find it in their hearts to work car customization into the game very soon. Community-created vehicles have already been equipped with a primitive customization system. Modification in general is VERY simple. You can edit the files freely. For someone like myself who has no modding experience, I have managed to download all of my favorite tracks and cars with ease. I've also been able to edit the .ini files to customize transmission, suspension and engine files to my liking.
This game will rise to the heights that rFactor and Live For Speed achieved years ago. It will surpass them (in my opinion, it already has) in terms of community involvement.
Whether you love to race, drift or simply drive. This is the game you want.