From my experience with GT5/past GTs... basically you just buy the necessary car that meets the requirements for each series and play through them so you experience a wide range of cars and races. You win tournaments to earn the money to do this. Though yeah, GT5 is very Japanese in the sense that it's insanely menu heavy. Also, if your car sucks compared to the rest of the event, then you weren't looking at the specs when you bought it! My knowledge of cars is only through racing games but I can still pick cars just fine. Also, try used cars, try saving up your money, try selling similar cars, etc. It's not that hard to earn money to buy what you need.