Soldato
You don't need to buy them all, only what you want to race. Once you get to D licence the best car you can buy to get you access to the most series is the Ferrari 488 GT3 (not GTE!). It'll get you in to Ferrari GT3 Challenge and VLN Endurance. Once you get to C you can use it in IMSA, and finally a B licence will get you in to LMS, Endurance LMS, VRS and Endurance VRS.
Good advice, one minor correction. The Ferrari GT3 is no longer usable in IMSA, it got dropped to allow the Porsche 911 RSR in due to car limits. GT3 cars for IMSA are the Mercedes and Audi.
For VLN you need to own both the Nurburgring Nordschliefe and the Nurburgring GP to enable the combined layouts.