It's not like you have to HAVE every single car in the game to appreciate it and its different racing styles. And no, there are not "millions" of different, car specific competitions in any GT. Here is the Race Events list from GT4:
GameFAQs said:
a. Beginner Events
b. Professional Events
c. Extreme Events
d. European Events
e. Japanese Events
f. American Events
g. Endurance Events
h. Special Conditions
i. Driving Missions
j. Manufacturer's Events
I. America
II. France
III. Germany
IV. Italy
V. Japan
VI. Korea
VII. UK
Bare in mind you don't need a specific car for each one of these to partecipate (Endurance for example).
Then again, we know shit about what exactly you are going to get when you buy a track. What do you get for buying Twin Ring Motegi? Do you just get the Road Course or you also get its variations (East Short Course, West Short Course and Super Speedway)?