GT6 offline at least you can buy and pick any car you desire to drive, even offline. GT5 had events that required specific cars only sold by Used Car Dealership, which was very annoying. I know this happened on GT4 and older games too, but the problem is that on older games, most of the cars were obtainable by simply buying on the dealership, UCD had exclusive cars too but they were very few. On GT5 however, the majority of the car roster is only available through UCD....
When GT5 was the current game, you could trade or borrow cars with your friends, making this task a little easier, now, or you hack your savefile to pick these annoying cars, or you test how far your patience goes to obtain these.