Assetto Corsa Competizione

Assetto Corsa Competizione (ACC) is the second entry in the Assetto Corsa series. And though the executable may be called AC2.exe, it isn't exactly a sequel.
It's a spinoff that trades in loads of content and modding abillities with a better physics and tire model and features that arguably should've been in AC1, such as changeable weather, a day/night cycle and better Online functionality.
Contentwise, it "only" features the 2018 and 2019 Blancpain GT Series aswell as the 2019 Intercontinental GT Challenge as a DLC expansion. In the future the car roster will also be expanded by another DLC Pack with 10 GT4 cars.

If you're only into simracing to drive shitty road cars down japanese mountain roads or absolutely hate modern GT3 and GT4 class cars, then ACC isn't for you.

Where to buy

You can buy it and the DLC on Steam

Custom skins

The only way to mod this game so far is with custom paintjobs. Though the game already comes with a very basic paintjob editor there is a way to make your own like in most other simulators.
To find out how, go here