It's a GT/Forza-ish style sim, though it's much more focused on a racing career than buying cars and upgrades. All the cars are available for non-career racing from the word go and there's no economy in the career; you're just trying to make your way through driving another team's car, earning points, looking to score new rides in bigger series, catching the eyes of sponsors, getting invited to events, etc. You can tune all of the cars to varying degrees depending on how they're outfitted (for instance, a go-kart and an F1 car will have different tuning options available) and the tuning options seem to be somewhat more detailed than GT or Forza, but similar. You don't buy different parts or customize paint jobs or wheels, though you can swap different types of tires on depending on conditions. Tire physics and driving dynamics are quite good, and wear and tear is much better than expected (sim damage causes real problems, and fuel depletion is a factor in longer races; you can even choose to not take a full fuel load to save weight). Day/night cycles and weather are very slick.
As far as the customization goes, here's what all you can do:
https://www.gtplanet.net/forum/threads/ ... ve.327780/